I can't usually find a cross that fits well (It happens once in a while), and I usually use some combination of scotch tape and non-hardening putty to make it seal nice.
As for the pipe, I'm mostly amazed that it seems to have an ID of 5/8". I've never seen anything like that for copper tubes, but as you said it's "repair" tubing so I guess its intended to slide over 5/8" OD copper tubes and get soldered together. I often have trouble finding 3/4" CPVC that fits over the EMT--it sounds like this copper tube is a bit smaller on the OD which makes buying the 3/4 CPVC bow arms much less of a hassle.
I also wouldn't be worried about said copper tube getting ruined--it doesn't have any holes in it, and it's being held in a way that can't properly deform from bending until after the whole cross assembly has exploded. So long as that doesn't happen, I think it will be OK. The EMT was super overkill in my version, it was just the only metal tube I knew that fit 1/2" CPVC on the inside and 3/4" CPVC on the outside.
Definitely a bit of a hassle with the 3/4 CPVC. I used the one from McMaster, took quite a bunch of hammering to get them to fit. Since I don't have a CPVC supplier locally, I have to work with what I can get, instead of finding one that has loose enough tolerances. I think I only got it to nest inside maybe 3/4 as much as in your write up. Works well, though, shoots at least 200 fps.