Considering my lack of knowledge in that department I feel like out of these two ninja seems better just due to PvE factor. In its case I also have a freedom to swap in between specs to see what I prefer (given your description and what I've read online).

Maegu seems interesting too then, both of her specs sounds like my cup of tea.

I hope I didn't confuse anything and if you feel like I did, please tell me to clarify the misunderstanding. So far I feel like DK, Ninja and Maegu have two specs that sounds interesting to me. Sadly there is only one season spot at once. I know, I could technically make a trial character but to be fair I value opinions of more experienced players over my blind testing that might take ages and probably won't give me answers I seek. On a positive note, 3 classes to go with through trial is already a better thing than going in blind.

This sounds interesting, how's the skill ceiling on Lahn? I saw some Ham454 montages with her and wondered if it's just farming mediocre players or the class has so much space for player creativity. How about her horizontal mobility? Or is the flying stance so easy to convert to that she doesn't bother with ground movement during fights?

Awakening Musa does ok against both evasion and DR, Awakening Maehwa struggles killing high DR targets.

Regarding high apm: https://www.youtube.com/watch?v=K05DPd6gBQ8 
https://www.twitch.tv/armintf/clip/PreciousColorfulOwlSuperVinlin-0gcTZJjrJwR67zyB 
https://www.twitch.tv/armintf/clip/BreakableGoodEmuBabyRage-HgVGZf3IJdwcySyk
(remembered Elmermol as I finished writing this so not gonna edit it and just gonna the video to the list)
I couldn't find a video of Armin grinding in a short search so those clips gotta do. grinding you use even more APM because for one you don't get slowed during grinding and sometimes during PvP you let animations play out more because of protection and being able to react to the enemy better. During PvE you pretty much perma DPS no break and you often have a 30% attack speed buff up.
In those videos you can see the inputs. Hotbar skills dont show up on the Keyboard Input UI part though.

For movement you can have different skills all on a different cooldown, but there is also a stamina as a resource. Some classes can spam some of their movement skills while on cooldown but they take additional stamina. For some classes like succession zerker stamina doesn't matter at all. And some classes have relatively low cooldown on their movement abilities but high stamina cost so you run out of stamina.
A lot of classes have really really fast movement. Some of it, while not exactly teleport blink movement, is still fast enough that it is unreactable. There is also instant teleport movement though, the worst contender for that at the moment is awakening Maegu which has a quite long range ability to instantly teleport into your back with a CC.
Succession DK has an ability called airstrike which is unreactable fast decently long range and can CC in the back. Succ DK wants to disengage sometimes and wait for CDs or wait for an opening.
Awakening Ninja usually wants to stay quite close to the enemy at all times, always within range to grab which creates some passive pressure on the enemy.
In BDO some classes don't even need to try to be permanently protected which forces the non grab class depending on the situation to wait for the enemy to engage and make a mistake. In those situations you can't actively do something besides waiting which definitely isn't fun.

For Season PvE clear speed doesnt matter much, almost all classes (besides Shai) do just fine.
It starts mattering more once you get towards mid or endgame.

Awakening Ninja is definitely below average in PvE and, while succession Ninja is quite good, it requires really really high APM. Also playing a spec in PvE helps in PvP because you will know all your cooldowns by heart, you will be better at judging the AoEs of your abilities, you get used and learn the muscle memory of skill cancels and chain, etc.

So far the classes I would suggest you to take a look at would be Succ DK, Succ Maegu and Succ Maehwa.
Succ Maegu is a good grinder because big AoEs and decent damage. Succ Maehwa is good because decent aoe and good damage. Not sure how good/bad succ DK at grinding is but I don't hear much good or bad stuff about it which should mean she is around average.

Ham is really good at always catching enemies off guard, he knows where to engage when. Succ Lahn can definitely fly a lot, but you will also use ground movement and you will have to stay on the ground for while from time to time, as using the flying skills costs stamina and blocks stamina regeneration while flying.

Pretty sure the only people who think the 10 seconds SA buff is a good mechanic are only Nova players themselves. Because that buff is so abusable it is so annoying to fight against.

Nowadays theres no class anymore that runs out of stamina except for high movement spots. I already wrote something about Succ and Awakening Ninja PvE. Succ Kuno PvE is quite good around the same as succ Ninja but way lower APM.

So how do these players actually manage to main these characters, you've mentioned a really important thing:

Do I guess correctly they're actively grinding with these high apm classes? Or they're just doing that often enough to get good enough feel for a class and then actually swap for grinding purposes? Btw. how does it work with addons? I suspect there are different ones for PvE and PvP (probably even different ones for duels and zerks) - if I want to approach a class like that and engrave all cds, combos and cancels into my muscle memory, what path should I take building my class?

Yeah, by blinking I don't necessarily mean teleport itself, any unreactable fast movement is basically a blink for me.

Which ones out of these 3 would you call to have the biggest skill ceiling in PvP and PvE? Basically which ones have the most room to present your time and understanding of a class itself. I do get that in PvP probably everything allows for that but I just wonder which one has the biggest potential (mostly speaking about class design, not meta).

Sorc main since day 1, never rerolled off the class.  Sorc has, probably, the best short range / in the pocket mobility.  Her blinks render her invulnerable during the animation, so you can pull off some sick outplays while in melee range.  Because of her top tier short range mobility and abundance of iframes, she has been one of best, if not the best, "1vX" classes in PVP, able to fight multiple opponents alone or handle herself well in the middle of zergs.  The cost of this, however, is her lacking long-range mobility and dependance on stamina.  Often times, once a Sorc has committed to a fight, she's in it since she will consume stamina, rendering her already limited disengage options (as she can't cover large amounts of ground as quickly as other classes) even less effective.  Disengage for a Sorc often requires being sneaky about it, like use Shadow Leap (her longest range blink that isn't very far) out, into Nightmare (her invisibility move where you move very, very slow while invis), inch your way behind a rock or a wall, into block to regen stamina, and use abilities to get away, can't just bunny hop in, spin to win, and run away like a madman like Succ Berzerker can, for example.  Stamina management is a key skill to learn on Sorc in PVP.

Each iframe chunks your stamina by quite a lot, so learning how to manage your stamina by weaving abilities between them to stay protected and regen stamina, while simultaneously going for offense and defense moves, knowing which abilities to use and when, takes time to learn.  She has various animation cancels to make her even slipperier and in PVE, her Awakening stance has various animaiton cancels and movement ability rotations to maximze efficiency,  This also takes time to learn, IMO, longer than most other classes.  IMO, her pack of pack mobility isn't that bad...spots like Centaurs or potion piece spots, she not the best in terms of pack to pack mobility, but she has decent AoE, a ranged nuke, and her mobility in these spots isn't too bad, IMO.  Like, I get around well, but it isn't like Maewha where you just hold down RMB, you need to weave abilities together.  She does do pretty well in mid-late game spots where pack to pack mobility isn't as important.  Spots where she can sit and perform full combos is where she's best at and her Succession stance offers a handful of frontal guards to help grind spots with harder hitting mobs if you need it.

Overall, Sorc is my favorite class and she just feels like home for me.  I enjoy both specs, but Awakening is my favorite spec in the game.  Other classes I found fun are Awakening Mystic, Awakening Striker, Archer, Succ Ranger, Scholar, Musa, and Maewha.  I think that's the most important part is finding a class that can feel like an exetention of your own body, to a degree.  IMO, pick a class you find appealing and stick with it.  Get good with it and you can't really go wrong.

Some people just aren't bothered by high APM. Some people hands/wrists start hurting after 1 or 2 hours of high APM grind. I personally dont mind and have no issues with high APM grind. Probably more than 90% of people can't do high APM for longer times though.

Back when I used to PvE on awakening Kuno I knew all my cooldowns by heart. After I started PvEing on another char I started needing to look at my cooldowns in PvP and sometimes messing up my cooldowns despite looking at them. I stopped PvEing on awakening Kuno because it is atrociously bad in PvE to the point where good classes get 40% more loot in high end grindspots.

You have 2 different skill presets unlocked for free and you can buy more skill presets in the Pearlshop. You can swap between skill presets with a 10 minute cooldown. This includes skill add-ons (and everything else). You can save a skill preset before you choose skill add-ons and then you can always load an empty preset. You can also change a skill add-on at a skill instructor for 1 Memory Fragment which is around 4 million silver on EU. You can make 15 million silver per minute with full PEN Tuvala gear, all adventure logs, PvE crystals and lightstones, and proper PvE damage and drop rate buffs. So changing skill add-ons isn't expensive.

For character builds you cant really mess stuff up. You have seperate PvE and PvP Crystal and Lightstone set which you can swap freely in between. Ideal PvE gear is Blackstar mainhand, awakening, offhand, kutum offhand is fine as well, accuracy gloves, DR boots. For PvP DR class you slap on a nouver. For PvP evasion class you use evasion gloves and boots. Accessories for quite a while.

Skill ceiling I can't judge at all. I haven't played these classes. My knowledge is only from fighting against them. I would like to say if you're playing a class which needs to wait for its opening to engage then you need a little more game knowledge than a class which is constantly throwing out ranged damage and kiting away, but that also isn't fully correct. Imo succ Maegu is one of the easier classes though. If you want to you can try out Ninja and check if you're fine with the high APM.