1991 Chevy Cheyenne Oil leak/starter problems

1991 Chevy Cheyenne pickup, 4.3l 6cyl - truck wouldn't start, sounded like typical starter spin - looked under hood saw arc near manifold and solenoid - pos cable rub. Crawled underneath to remove/repair cable and found one of the two starter bolts missing and starter loose. I was last to change the starter and bolts were torqued per specs.

Now the kicker - there was fresh oil all over the starter and buildup on the transmission case. Further inspection shows oil on top the engine as well, around the carb. Apparently, I have a leak or two.

Figuring the main culprits are rear seal and valve cover? Is there an oil sensor on this model that could also be suspect? Is it likely the oil leak + vehicle vibration loosened the starter bolt causing it to fall out?

I am going to start by degreasing and conducting a more thorough inspection but always value the information I get on this site. Your opinions are appreciated.

**EDIT** Son just advised that the oil pressure has been running high for sometime now. The gauge is a low to high only and he said it runs pegged out unless stopped. Of course, since it gets him away from house he didn't care to mention it. This makes me suspect the oil pressure sensor/pcv valve.