I'll take a shot but Ron and/or Dan might be better for this one.
As to the overheat, I think we need more info. 170 is not overheating. 180 is not overheating. It may be getting warm but certainly not overheating. Did it get warm at idle, at speed, just after a long run or after you shut it down and then restarted?
As to the 220 spike, it sounds like the belt that broke must only run the circulation pump? At least that's how I took it. I would think if that's the case, it may spike momentarily if the boat was on plane when the belt broke and then brought down to an idle which sounds like what happened. I would think the engine should get enough water flow from the RWP to keep it reasonably cool as was stated at an idle.
If that is truly the case I have a hard time believing a momentary spike in the temp would hurt it as long as came back down right away at idle, but I could certainly be wrong.
As to the manifolds in salt water. Yes, they don't last long then. I would say that 3-5 years would be about correct. However if it has truly only been in salt once, than no way they should be bad. We don't have any history of the boat though unless you can shed some light on that.
I would question why the manifolds need replacing. They measured the port manifold at 130 and the stbd manifold at 145 with the engine reading 165. If they aren't leaking and you don't have water in the oil, I wouldn't touch the manifolds yet.
I would verify the impeller is good, replace the belt and verify what temp thermostat is in there and if it's good. Once we know when it starts getting warm as I asked above, we can better determine where to start looking for an issue.
As a general rule, overheat at idle and cooling off at speed is normally a slight leak on the intake side of the RWP or any of those associated hose connections, cooler or strainer.
Running cool at idle and then getting hot at speed is normally an obstruction or questionable impeller.
Temp creeping up to 170 at speed or a during a hard run if you have a 160 thermostat is perfectly normal.
Shutting off a warm engine and restarting after a few minutes the temp could be well over 180, but it should back down right away. Again, this is normal.
Some more info would certainly help.