I have not built any pond structure with plastic tubing of any type, but I have done plenty of mickey mousery with various plastics. I think your wire solutions to hold the tubing firm will work for many many years. I can't speak for PEX's UV resistance, but then again, once it gets submerged and algae covered it will see much less light.

I would experiment with heating the tubing with a small propane torch at the PVC just enough to make it pliable and then push it back on itself to create a swell (or a goober bugger), then hold it until it hardens. The only draw backs would be if the plastic gets brittle after it cools and hardens, and it could get a little unsightly. Obviously, brittle plastic will not take any flex and the experiment would be a failure and it's cosmetic appearance will not affect it's function, only the pre-submerged photos.

Stainless steel hose clamps might also be an option, but at a dollar or so a piece, it might push the budget.

Fish on!,
Noel