Hello! I don't have many answers to your questions but I can give you a big thumbs up!
Looks like you did good research and I'm fairly certain you will build an awsome machine!

Ballscrews with 10mm pitch is good for router speeds ie wood and plastics. If you were to use it as mill for steel and such then 5mm pitch is better.

If ballscrews are bent then maybe you could straighten it. Much worse if the end machining for the bearings are off center :)

Steel is good for gantry and plates as well as aluminium, you just don't have to use as thick stock!