Why didn't the Swiss test the F-35C USN carrier variant but the F-35A Air Force version? They have very high and short fields and limited hangar space inside their tunnel hangars. So a heavy duty gear and foldable wings might come in handy? And the F-35C got reworked and updated.

The F-35C hasn't had any mods that the other variants haven't (none that are applicable to land-based operations at least).

It might be more suitable for some of the reasons you give, but it is more expensive.

Isn't the buy-back due to FMS and ITAR regulations ie stopping the user from selling the technology to a third party?

The Swiss could sell the F-5s on, but would need US permission. The US doesn't need to buy them back if it has no use for them.

Wouldn't some pre-owned but well looked after AD capable (only) Typhoons fit the bill? Provide some cost recovery for the Home nation to fund some latest-spec Typhoons to replace them......

If Norway could operate it's F-16 over mountains and very hostile terrain it might in fact work for the Swiss as well. Plus the Swiss have a history of buying maxed out and perfected end of the line aircraft for some good price. For pure air superiority missions the agile Falcon might still be good enough. However it seems to not be included in their final selection.

I don't know anything about these F-5s but knowing that they were operated and maintained by the Swiss, I could agree with your comment. I worked with an operator in the US for awhile at a time when the company was purchasing several retired DC-9s from the now defunct Swissair. The Swiss aircraft had been so well maintained and in such good shape that the company couldn't understand why the Swiss had disposed of them. We quickly realized that it was the Swiss-quality/culture in action. The guys in Flight Dispatch boasted that the used Swiss aircraft were the most reliable in their fleet.

Swiss DC9s
 

Flew them and was shocked at no carry forwards in the tech logs after coming from a flag carrier where the captain spent the best part of ten minutes reading through the tech log in first class with pages of carry forward defects often with no spares available.
Nothing but praise for the Swiss engineers but a pity they were overruled with the entertainment system for the MD11 and outside contractors were used. Their winching the system cables through was the reason for the loss of SR111.