Jozz,
Fhwhooaah!!!?
This, to me, is the almost inevitable problem with anything made of plastic: it eventually becomes brittle and breaks, all by itself: household items, motor car parts and....accordions!?
I have a 30 year old Bugari Champion Casotto .
This has a plastic part (slide?) in the coupler mechanism. So far, it has broken twice. The last time, it cost me A$500-00 and a five month wait for parts from Italy!
My oldest accordion (70 years+ Busilacchio) has shown no problems with the coupler mechanism ever: no plastic!
( Same with my other accordions)
Even if you strengthen the anchor point, by reinforcing with metal, unfortunately your anchor points may well be brittle themselves: check around. Overall, epoxy plus metal ( or wood?) could well be the best option ??