Just the washer seal under that nut.  Easily replaced.  Not critical,
doesn't look like it's leaking too much there.  Check first to make sure
it's tightened down solidly, but don't crank it TOO tight.

Not really.  Oil leaking down from the above washer is spreading out
when it hits that flange and collecting dust and dirt.  Happens on most
engines over time.

Once again, check the tightness of the nut where the leak is, and
replace the washer and seal if necessary, then give the engine a
cleaning with the cleaner of your choice.

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under the metal cup washer is a rubber biscuit sealing washer. id take
the nut and metal cup washer off, and see if the rubber one pops out.
pop it out, clean, examine. if it looks ok, put it back, retighten
everything nice n snug. but im anal like that :)

or just snug the nut down a bit and wipe the residue clean, then keep an
eye on it.

nope. not soot. just plain old dirt. probably came through the grill,
over the valve cover (the back has fine dust on it) and settled there in
a nice air-free pocket.

thank YOU for taking the time to post pictures! its one of the great
things about the internet.

on second look, it appears the other one has a small leak as well. oil
seeps down, and into the crack where the dirt is attracted to it.