JJ:

The 22-23 deg total variation (12-13 before to 10 after) that you have observed is quite unusual. Even if the chain and sprockets are very worn, and the tensioner is tight, the marks on the rim of the dampener will be consistent from rotation to rotation, albeit retarded. The engine has always been turned in the running direction (CW looking at the front of the engine), yes? Turning the engine "backwards" with a loose chain can lead to jumping over a tooth.

If you can, hold a small mirror behind the back of each camshaft and read the two-digit code # on each. The numbers will be in pairs, left/right, example: 62/63, 68/69, 70/71