With today’s unleaded gasoline carbon buildup is not an issue anymore. That mark is harmless & likely from an over torqued bolt. (**** happens).
Were there any issues while running? I would put it back together, don’t look for problems that don’t exist.
It’s likely you’ll create an issue...
If it was me and there was no obvious issue I would swap temp probe positions to determine if the temp probes are accurate. If readings are the same then ther may be an engine issue.
I have a ZD1211 basically the same as yours. Never had a caster on any mower Kubota/Scag get stuck as you describe. I would like to remind you that the casters should not be supporting the full weight of the deck. On a hard level surface they should be approximately 1/2 inch off the surface...