I have seen oil degrade in a matter of a few thousand miles in big diesel engines but not a small capacity modern petrol engine....seems rather odd and worth getting a sample analysis of the oil quality
If you try applying moderate force to see if the glovebox lid will flex enough to release, but think this may damage the catch and could be expensive to replace.
I think the best method would be to request a locksmith to unlock it with a skeleton key as the glovebox would more than likely not have a high security barrel fitted.
The above diagram shows the timing marks for crank and cams.
You would also need to check the tension and condition of all components to see if serviceable if not then replace all parts
Check also the air con pump and other ancillary components to ensure they are turning free.
Maybe worth trying locking and unlocking as many times as possible to see if it will eventually unlock.
if not, then it should be possible to remove the doorcard from inside with little or no damage, depending on how adept or heavy handed you are 🙂