1. She was buying time for Artoo to escape with the plans. The Imperials were shooting any departing escape pods with life signs aboard to begin with.

2. Arguably, the trash compactor system is automated. Otherwise, somebody would have overridden Artoo's ability to shut it down, let alone open the hatch.

The homing beacon plan could have been hatched and implemented only a short time before the gang got back to the docking bay. The ship was lightly guarded, and the troops present didn't interrupt Vader's duel. Those particular troops may have been the only ones with specific instructions not to hit anything.

3. I'm going to go with about the size of a small moon. ;)

4. The theories of potential damage to Endor has been addressed elsewhere, but I'm too tired to look it up right now.