If after, has the poly-belt shredded again?
I have a suspicion that a ploy-belt getting under the timing belt might cause the cam timing to slip a bit...

Have you bothered performing a compression, or cylinder leakage test, to check for any leaking valves?
You can perform a compression test with the cam followers, or cam out, if you don't yet have a belt fitted.
Normally the valves don't bend, but one member did suffer that, and I recall, the damage to your engine was with it running at speed.