1) YES
2a) If there is a carb issue, remove them, clean them, and set them to factory specs.
2b) NO... if you go pre-mix, do not try to "Adjust" the mix in the tank. That will lead to a dead engine. If you think it's wrong, drain the fuel, clean the tank, and mix fresh fuel.
3) There is no load OUT of the water. I have said this before, so this will be the last time. You can't check, tune, or adjust Anything fuel related on a jetski, while it's out of the water.
Also... the plugs are going to get dark out of the water. Since there is NO LOAD on the engine... it can't burn the fuel.
The Run-away issue may not be an issue. But you don't know until you put it in the water.