Yes expert AI. No I dont think so, but as in any mod, I cant be 100% sure.
You dont. You send your 18w at the start and build them up to 40w through all the army xp you get from fighting and LL.
Yea Guderian is not good since he starts with 2 earned traits (the other 2 are personality traits which do not count/matter when talking xp gain)
Not sure what you mean by grey or blue traits. EDIT: ah I think I understand now. Blue are just those you picked and grey are those unlocked but you havent picked. They dont matter, not in this sense. What matter is the difference between gainable vs assignable trait. The wiki details which ones are which. IIRC, gainable traits affect further xp gain, while assignable ones dont.
You use the 5 army xp from Rhineland to spam 100 2w infantry, and when you have more than 120 divisions (excluding volunteers already sent) you can send 7 to nat spain, which is the most you can as it is further restricted by the amount of land they control
thanks - this way I got more XP on traits in 2 weeks than in complete SCW event before!
but I think it's really terrible that this is the only way to get any traits at all!
Last point is a bit to gamey for me - but at least you won't stop learning new things...