1. "Very carefully". I am assuming this is a "flat front" bus, no hood. There may be an access door on the front to allow access to check the radiator. You may have to open to "doghouse" (engine cover).
2. I'm thinking the engine air filter may be under the doghouse, follow the intake hose to find it. It'll be a rather large round filter.
3. Yes, that's the starter. It may be weeping oil that's simply leaked from elsewhere and this was the lowest spot before dripping to the ground.
4. As for the compressor governor, I'm not sure, I'd need a better view. I have a Bendix compressor, mine looks different.
5. Yes, twist those "tabs" 90 degrees, they may be a bit tight. Drain all the tanks until no moisture comes out, the "wet" tank may have a little oil mixed in. You may have an air dryer in your system (somewhat common), these need occasional service (replace the "cartridge" about once a year on a regular driver). Typically these will "purge" each time the air pressure builds to 120 and the compressor cuts out, with a distinctive "PSHH!" hiss of air.
6. Yes, that looks like the power steering reservoir, the smaller cap should have the dipstick. Many are a rubber friction fit, it may not unscrew, just carefully pull up, try working it side to side a bit.