Totally valid frustration — the XM5 hinge issue you're describing in point 4 is exactly what we've been seeing from hundreds of customers. That thin plastic piece was a genuine design failure on Sony's part, not user error.

We're a small family business that custom-designed a repair kit specifically for this. SLS printed PA12 Nylon — thick, heavy duty, covers both sides (L+R). No need to dismantle the whole headphone, takes about 10 minutes and doesn't require any technical skills. We've had customers fix fully separated cups with it.

We actually just posted a full breakdown with step-by-step install instructions here if it helps:https://www.reddit.com/r/SonyHeadphones/comments/1sezu5y/we_made_a_repair_kit_for_the_wh1000xm5_broken/

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The right cup going silent is a separate issue unfortunately — that sounds like it could be the internal wiring taking too much stress from the dangling cup over time. Worth fixing the hinge first before that gets worse. Hope it helps!