What is "P-lead"?
Craftsman does not offer an online wiring diagram for the model you gave, but looking at the diagram for 917.271022, I see that model has 2 operator presence relays.
OP relay #1 kills the engine by grounding the magneto wire.
OP relay #1 is what kills the magneto when the seat switch reports an unoccupied seat.
But, because the relay kills the magneto whenever the relay IS NOT ACTIVE (default position), a defective OP relay #1 can also cause "no spark" even if the seat switch checks OK.

Looking at the 271022 diagram, it appears that removing OP relay #1 would prevent killing of the magneto EXCEPT by turning off the key switch, so I can't see much protection (as an interlock) offered by the arrangement.

But I also don't know whether your 271020 model even has wiring like the 271022. Phooey on Sears for leaving everybody in the lurch because of their omission.

Thanks Mownie I was looking at those, removed one relay and had spark, and all of the safeties work as they should.

Good! I suggest that you do not source a replacement relay from Sears (cost).
That relay is darn near available anywhere. Auto parts stores have them for much less. Just take the old one with you (or order from one of your trusted suppliers). It is a Bosch five blade, 30 amp rated relay. Used worldwide in just about everything.