Answers:
1) We had to use a 3-4 ft lever arm to remove the chuck from the rest of the gearbox. IIRC, we chucked up a large hex bolt, grabbed a wrench and a really long lever arm, and started turning. It eventually broke loose. I think we had to hold the entire gearbox and housing in a vice. (I know we took pictures, they're just not up on our website...)
2) The OEM motor for that gearbox spins between 15-20,000 rpm's. That extra planetary stage is needed to reduce the output speed to something usable on the drill. The CIM only spins around 5,000 rpm's, therefore the removed stage to achieve a similar output rpm. If your application requires that 1st stage in the gearbox, I'm afraid you'll have to engineer that yourself.
3) I believe you are correct. The tophat is plastic because it's easier and cheaper to machine it out of plastic. I don't think it really takes any stress at all.
BEN