Crawl up under it and look for signs of external leakage at the timing cover to block union. Everyone that I have seen leaking exhibited at least traces on the exterior surfaces... even if it was just a whitish trail.
Note the repair time guides will tell you the oil pan has to come off (the...
That filter actually has more than 1 number (look for pictures on e-bay)
Yes crosses to PF48 (which is the wrong filter for a Mustang) and Fram PH10060 (also wrong filter).
Probably a case of filter consolidation at the quick lube, but I'd stick the right filter on it.
That is a pretty special bolt.
Unfortunately I think you are going to have to grind or drill it no mater what. Gotta get something out of the way.