my mainhand ap is very low. I only use awakening and I am a pve player. is there a point in upgrading a mainhand?

Yes!! Awakening class will use 30% of total damage from mainhand. And mainhand provide you more monster damage AP as well. So for either awakening or succ class, mainhand still be a better choice to upgrade first 

They invented this system to force you farm 3 weapons regardless of your choosing!

It was to damn cool to have awakenings as separate skill tree, with no influence from main and secondary weps.

Technically yes AP, APP is shared like a certain % of your AP goes to AAP for awakening, some of your AAP goes to succession. Why not a 100/50 split for succ/awake who knows