Moving light fixture- what are these extra wires?
To answer your questions in turn:

A) Top right in your diagram (and in your photo) is the switch loop to your wall switch -- the white wire on that cable should be tagged with a piece of black tape to mark it as a hot. One of the other black wires is the (un-switched) line originating in your panel. The remaining cables go to other branch circuits.

B) Just put a blank round cover of the appropriate size on the junction box here after removing the light, so that the next bloke who has to work on the box can get to it.  Don't forget to remove the silly green pigtail connecting the nuts!