Decided to go for 1k lacquer and it looks great but it just hasn't got the durability I desired and is scratching too easily

I now have the dilemma of what to do next. I have been advised a 2k lacquer won't go over a 1k lacquer which means i'd have to strip it back down and re-paint.

Would the XCR resin go over the 1k lacquer if keyed first?

The trouble with going over a 1k lacquer is that 1k lacquers never properly harden. So you go over the top of it with a nice hard 2k or epoxy or whatever, but the 1k coating underneath still moves around and shrinks and outgasses etc, and it effects the 2k coating over the top of it. So if you are after durability you are much better off removing the 1k coating and restarting.

I thought epoxy had a certain degree of flex to it? How come it doesn't crack on materials such as wood that expand and contract?

Wood is a solid Material, 1k laquer isn't. The epoxy might glue to the very surface, but in the end, everything ontop of the 1k laquer just floats on the painted object.
1k rattle can laquer is really not a good idea for basically anything that needs mechanical surface protection.