be adjusted down to 140mm with a spacer?

Yes.

Yes (the 2013 ones can’t, but 2012 and earlier can)

Excellent, out of interest, can a 140mm fork be upped to 150mm by removing a spacer?

Yes 150 can go to 140 if dual air sprung, with a spacer.

If the dual air spring is a 140 max they can’t be extended, unless you put a new air spring assmbly in, and then the damper maybe specced for 140mm max so best to check everything.

2013 solo air revs can be reduced in travel, but again this requires replacment of the solo air spring assembly, we’ve got everything you need for all these jobs 😀