1/ With the exception of gloves: is it possible to buy clothing for, say, V3 and with a few clicks adapt it so that it fits an SP character that I bought here in the marketplace?

It depends on the clothing. With a lot of clothing, yes, that's how it works. (Though there may several minutes of waiting between clicks. ;-)

There will be a little more work to fit clothes with long skirts, coat tails, and the like. Because those parts don't really conform to the figure. Still, it's not too hard.

2/ Can this also be done when mixing earlier people, say, from V2 to SP, and/or from V3 to PT?

Yes. You can pretty much convert between any of the figures that are supported. And a lot of them are. I mean, really a lot!

3/ In case of yes: does this still hold for custom morphs like Thorne's for the PT or Blackhearted's for SP, or one I created myself?

Yes.