I'm sitting here thinking about this mysterious chain enlongation then shrinkage and have an idea. This may be way out there but here's my 'what if'.

What if the chain and sprockets had an uneven wear along the length of the chain and around the cam, crank sprockets???
Since the cam does not run at the same speed as the crank the chances of having both sprocket's teeth mated to the exact same links of the chain the next time it was checked would be vary rare.

It could be the first (and last) time you checked the stretch it was all mated up tight and then, in the loose position the second time.

Just a wild guess.

Advancing the timing causing the pinging to go away does sound backwards to me. Normally, pinging is due to injection timing being too far advanced.

Please do keep posting your findings.
Good luck!