Furnace continously shutting down due to lost or low flame sense (Rheem)
I'd definitely correct the slope regardless, you may be able to use a pry bar and shims if you're carful. The gas valve can only deliver what it's given. Gas line sizing is a pressure/distance/capacity/diameter relationship. @7"water column (normal delivery pressure, at least where I live) a 120mbtu furnace with 50' of gas pipe between the meter and gas valve will require a 3/4" black iron gas line. 50' of 1/2" will only deliver 95mbtus. You have to allow for anything else like a water heater or stove. Tankless water heaters are usually 150-200mbtus.

It's hard to say what the problem is without seeing it, but it seems like you've explored other potential problems already. It could be a combination of issues too.

P.S. Pipe size is measured by the internal diameter. 1/2" pipe is actually about 3/4" outside diameter.