From what you describe, it still sounds like the seat safety circuit is still the issue.
The ETR (Fuel) solenoid is powered through 2 seperate circuits; either the traction switch circuit or the seat switch circuit.
The power feed to the ETR (Fuel) Solenoid runs through the traction switch when the transmission is in neutral. Once you step on the pedal (forward or reverse) the traction switch will open and the power would then be fed via the seat switch circuit. Since the tractor starts and runs when in neutral, we know this circuit is good. So that leaves the seat switch circuit to review.
The electrical circuits are all laid out in the Service manual. If you do not have one, you will need to contact your local distributor to get it. We do offer free manuals on our web site, however the GM 455 is out of production so the manual is no longer offered.
As far as the snowblower goes, I do not know how it may interact because you did not provide the manufacturer. It may fit on the 455, but we (Toro) did not produce a blower for that unit. One question I would have in relation to the blower is does the tractor shut off with the PTO engaged or disengaged? If it shuts off in either position, I would then direct you to the tractor to discover the root problem.
I hope this at least helps you get a start on problem diagnosis. You have "jumped" the seat switch, but now it's time to get diagrams and the DVOM out to dig further.
Toro Commercial Service