Was it calculating correctly and then stopped?
Out of curiosity, what are the average and instant MPG readings?
It should reset automatically, but wonder if resetting the avg MPG would recalculate the fuel range
It's still possible to be a faulty gas sending unit, have you ever run the tank to...
Could be ignition issues that are triggering codes.
Given the symptoms, have you replaced the wires and plugs? Also the ICM could be at fault since the problem appears when the engine gets hot. Electronic components can be fine when cold, but malfunction when hot.
I would not tighten to much over spec, snapping a bolt would make me cry!
Would not recommend removing and reusing the gasket, it may be damaged
Did you thoroughly clean the mating surface?
Is this a paper gasket or metal with sealing bead?
Checking high and low pressure readings is helpful when diagnosing a/c problems.
It you discover the a/c compressor is intermittently cycling off, check the connection at the compressor and swap or replace the compressor relay. Sometimes relays malfunction when they get hot.
Hopefully it's not...