In my experience pressing the reject button should just cancel the current song.
Songs should play in the order that they were selected.
Realy not sure how many selections can be made at one time. Mine regularly plays 4 or 5 selections without any errors but does occasionally either miss a track ie plays 3 when 4 were selected or replaces one of the selected tracks with a random track.
Yes the built in speakers will still work when the auxiliary socket is used. If a Bluetooth transmitter is plugged in there is a delay between the Bluetooth speaker and the built in ones which causes a echo effect. I fit a switch in the speaker circuit so I can cut the inbuilt speakers if I wish.