Here's my take:
1. 65536 is the CWID. Unitrunker handles that by assigning it to an invalid TG, which is what 65536 is.
61120 is a school police TG, Irvine Valley College, I believe.
2. I have no idea. Radios cannot/ should not share ID's
3. Most likely what you are seeing is radio techs working on radios. That's my best guess anyway. Next time you see this, see if the same radio comes up on TAN-CALL and RED for a radio check.