Pex bend supports keep breaking
I think you've hit on the problem. I'd carry a bucket of hot water with me and dunk the segment to be bent for a few seconds before doing so. Pex is really stiff when "cold" (below 60&deg; F or so). It should then bend more easily and somewhat hold the shape after it cools.

Use plenty of pipe hangers adjacent to the bend support to, er... support the support. Alternate sides as appropriate.