Check the wear plate and gaskets to the lower housing closely & 200* won't hurt a thing for that length of time, I've had no more issues with aftermarket Sierra impellers than with OEM, there can be a bad lot in either. Make sure to grease the cup well and rotate the shaft clockwise as you put the cup / upper housing over the impeller & you should be good to go. If the cuts were on the top of the impeller you might of cut it or started it on the edge of the metal cup or not. Your problem seemed to be evident when you pull it apart but many times with Gen1 drives that tend to overheat at trolling speed & everything else is in order the lower housing is normally the culprit.
As you said> Loosen the cap with the motor running to bleed the air as it fills from the reserve tank or the quicker way is to pull the cap and fill the tank close to the top and bleed it from there the same way.
This is the advice I seek:
Is this a serious problem needing immediate attention or can it wait till July/August?
(will be a long wait for service now anyway)
**It should be addressed asap
When these fail is it gradual or all at once?
Will continuing to operate this way damage anything other than the dog clutch?
**Damage to both the clutch dog & forward gear is happening every time you shift it in to forward.
I would guess it's lower shift cable & or worn shift yoke or splines down below. At any rate addressing it now will probably save you grief & money in the end.
I'm a little older too but that Sea Star system is more than adequate to steer/control both motors & will work out well for you with the right link between the two.
It's normally open on or by the 1st of April, but with what has been going on this last couple of months they have just started on it. I would think it will be open next week some time.