You are right in all your assumptions. If it is a suction line blockage then whatever is causing it got past that valve. Doubtful I think. Maybe my next step would be to hook a length of 1.25" poolhose right to the pump and bypass the suction side plumbing entirely and see what happens. 30' though would be substantial flow resistance with that type of hose and kind of expensive. Since PVC is cheap, what about jury rigging 30' of 1.5" PVC pipe between the pump and pool. You don't even have to cement the pieces together. Cheap vinyl electrical tape (Home Depot 70 cents a roll) seals the pipe joints very well temporarily. Take it apart when done. All the pipe, unions, adapters, tape, would be under $15. That would definitely prove if something got in the pipes.
Did you try the filter valve in closed position? Also, does it have a 'bypass' or 'recirculate' position? Try that just for kicks too.
I'll keep checking back and thanks for the comments.
Al