My living room floor is the basement ceiling. I'm planning on installing pex between the joists using plates. There are lots of wires running along these bays and also crossing them. Some will end up in the heated cavity between the insulation and the plywood. Anyone know what sort of temperatures romex et al can tolerate for years on end? It's not really practical to move the wire.

The conductors in romex are rated at 90c (194f). Your plex temperature shouldn’t ever be any where near that high. Also romex is not plenum rated. Which means it can not be installed in an HVAC air stream. But I don’t think the plex insulation envelope in this case qualifies as a plenum. So you are probably ok.

However personally when I retro fitted my home with radiant I avoided having any romex in the plex insulation envelope even if it meant less insulation in some small locations.

I’d recommend you install at least some insulation between you plex/plates and the romex. Plate installation does not require an air gap between the plates and the insulation. For final peace of mind call your local electrical inspector.