OK, I'll play along.
1) they had to be not be utilizing or paying attention to the GPS
Unknown at this point.
2) they had a visual approach, but they still could have had the localizer frequency set, but did not
Unknown at this point.
3) The tower at Branson either did not have radar surveillance, or if they did, the controller was not paying attention
Unknown at this point.
4) the pilot not flying did not pick up on the error
Unknown at this point.
5) someone in the last 15 minutes during the approach just happened to activate the pilot controlled lighting at Clark field on 122.7.
Unknown at this point.
The "Swiss Cheese Model" also known as "cumulative act effect" was developed by James Reason and Dante Orlandella (University of Manchester).
There is a lot written on it and I spent a great deal of time using it and doing analysis as part of Aviation Safety at the MMAC in OKC (FAA Academy)
In order to do an analysis you have to start with facts, not speculation. Speculation will only skew the results.
We need to know the following:
Aircraft make and model? I realize it was a B-737 but which variant? How was the aircraft equipped?
Any MEL/CDL items in effect at the time of the flight?
SWA procedures for approach at night (visual)?
Duties of PF/PNF during approach (SWA procedures)?
Briefing. Was there an approach briefing detailing the arrival and approach? Where fixes identified, verified on the chart as well as the FMS? Was the NAV accuracy verified on the FMS? What is the maximum drift allowed? Where the Navaids hard tuned or where they auto tuned? Was the airport diagram referenced in the briefing pertaining to approach lighting, landing aids and exiting the runway? How were the respective ND's and PFD's configured for the approach?
ATC. What altitude does radar coverage go down to? Does the tower at Branson have a scope? Did any controllers notice the aircraft was off course? Was the crew asked to verify position?
Human Factors? How long had the crew been on duty? Was this their first?second?third?forth? sector of the day and what day was the trip sequence in (first, second, third, forth?) How much sleep did the crew have in the previous 24 hours? 36 hours? 48 hours? Was there a change in their schedules? Recency of flight (day or night) into this airport for either crewmember?
What do the Captain and FO's training records look like? Any problem areas?
Any other carriers or SWA had previous problems with this airport in the past? Ongoing issues?
This is just a small sampling of questions just to begin the process. I don't think anyone here (unless someone from the FAA or NTSB working the investigation) has these answers yet.