Relief Crew?
They may have been, but SW probably doesn't schedule their crews according to how many times they have been to a specific airport. If there was an "out of duty time" crew from the previous leg, they might not have been to Branson either.
Regarding the lights and tower call.
I could not find that either airport has pilot controlled lighting. KPLK might just leave theirs on all night. Some airports do.
The same might be the case for EBBG, especially if they knew there was a plane inbound (which they should have).
They were probably up on the tower frequency as a CTAF and that is why they called the tower.
It is a good question though, did they get an answer.
The Branson tower frequency may be remote monitored by approach control.