The sump nut is just that a nut! If the thread is knackered in the sump then tell them to run a tap through it for f*** sake! like 20mins to take sump off and do it!
Ive done oil changes on cars that are over 100,000 miles that are still on original sump with perfect threads.
Whoever took it out last time has over tightened it or cross threaded it WITH AN AIRGUN FOR SURE!
I hate reading stuff like this, and it makes me feel so fortunate to have all the tools and ramps to do it all myself
Just get main services done by a friend to get my book stamped.
Hope you get it sorted anyway mate (fingers crossed)