1. Are we supposed to be looking at the black area to the left of your thumb in the first picture? That is the exhaust area and it is normal for it to be nasty. A combination of burned fuel residue, water, and the oil that you put in the gas. The other possible area of concern might be the seal around the shift rod. Can't tell from the picture if it is leaking or not.
2. The pump plate is supposed to have a very slight upward cupping at the center. I can't decide, is yours pushed back down in the middle for some reason?
3. Drive shaft is completely normal and no it is not supposed to have an o-ring in that groove. The groove does serve a purpose that I won't take the time to explain, just believe.
Impeller looks good. Do you have any idea how old it is? If old, I'd replace it. But it looks like somebody has already been in there and put in a whole new pump. Perhaps he did that after it overheated?
I'm telling you that oil in the exhaust area is normal. And sometimes under certain conditions it can be quite a mess. Running properly, on a boat, on the lake it should be reasonably clean, but not clean-clean. It WILL be black and oily. That oil comes from the oil you put in the gas, not a leak.
If you are not getting water leaking into the gearcase, relax. But if you are getting water in, then find and fix the leak.