The problem is two fold. One, an unknown maintenance history. Two, a recalcitrant seller.
The issue was originally presented as a "soft wing walk". So concentrate your efforts there. Then proceed to stick the entire wing with a moisture meter and continue with a sniff test. Repairs (from meter prong holes) of fabric is minor repair, correct? If acceptable proceed with purchase.
Caveat, be sure moisture present is truly in the wood and not from occasional/standing water. Drain holes/grommets should be checked.
People do this every day in marine surveys. Then take that boat into blue water on extended passages. 100% certainty doesn't exist...not even for a moon launch.
EDIT: I believe UV-A and UV-B protection is warranted for a wing. If you mean applied directly to the structural members, I don't know. It makes sense that it would but I would want to research in order to select the best fabric covering.
Additionally, why not boroscope the wing? My brother has a 12' max length scope wand which is like 2 mm dia. with 17" color video monitor.