Ideally you want something with low friction, impact resistance and flexibility without fracturing. That’s why Nylon or PEBAX.
China is usually not good with RPs. They like to machine everything. I believe buying the expensive RP material is cost-prohibitive over there.
FEA is optional and later down the road. Sure you can get it to work on your prototypes but how many cycle times will it last? What about the load bearing aspect. At the end you may want some re-assurance that you won’t have to recall thousands of products due to a faulty buckle. Also make sure the factory doesn’t change the final material at the end.
Best bet is to do multiple iterations and send them out. You can always file or mod your parts to make them fit/snap, that’s what we do in-house til we get it right. Find an RP house that has polypropylene like materials or even nylon. SLA and Polyjet technologies have their own versions of flexible materials.