This game does have a learning curve, don't give up! You could reduce the game speed to ease you into this difficulty. All I'm gonna say is AFAIK, personal opinion and not thoroughly calculated.
None. The investment slows my progress and it's additional micro. (Sometimes I keep cattle in my corrals to produce a trickle of food, but I don't know if it's actually worth the effort.)
The part not build. (Workers didn't start yet = full refund, deleting existing building = all is lost.)
I hate formations with a passion so I wouldn't know.
Not at all; only bartering from the players affect the prices.
I believe the consensus is that walls largely aren't worth the effort.
Melee cav?