if you want to play as hayasdan good luck with that, there are severall other threads wich give you advice on playing as the hai
as said by the faction description they are nigh-impossible and aren't recommended even for mid good players so very good luck with that
other answers of your questions
huge unit size gives you a more real fealing and generally for me more men is beter also they are better for your unrest indeed because they have the seem upkeep as normal but more men=> more garisson bonus
also indeed the defense skill bonus doesn't work on arrows and it onlu works from the front and the rightso yes if you hit them in the left side this should have more effect

ok that was it I think good luck with your campaign and hoping you succeed