Bingowings said:

I think this is the cardinal error of ROTJ.

The Imperial Fleet there would be a tiny subsection of the ships available to the Empire.

The troops in the Endor land battle would be the normal troops defending such an installation.

There is no error. You just made it up yourself. There is no indication what so ever that the fleet stationed at Endor wasn't in fact just a fraction of the navy. And it is pretty obvious that the troops we see on Endor were not an entire imperial army but rather just a company-sized unit guarding the back entrance.