Hugh, you're right... no need to touch the pressure switch since you have shown it works. If it ain't broke, don't touch it.
Since disconnecting the ducting to the outside seemed to allow the inducer to operate at proper pressure, I would look at that ducting. Check all elbows and any flapper check valves. If it's clear and there has been no changes to the ducting, I can only suggest checking the blower wheel to make sure it does not have dust or other debris on the vanes.
10 years is a long time for a motor especially when operating in such a high temperature environment. Last suggestion is take the inducer assembly to an electric motor shop and have them bench test the motor. It seems the original horsepower is not there (assuming the ducting to the outside is clear). Hopefully the motor technician can explain what's going on.