10 psi is a lot of variation. The grid heaters certainly draw a LOT of juice, but it shouldnt draw down the lift pump that much, and either way it should stop when the grids do. The heaters only run at cold idle, below 15-20 mph. Unless the OP is driving thirty minutes at less than 15mph, something is wrong. I would load test the batteries, maybe one is getting weak, or maybe the grids are cycling when they shouldn't. Maybe you have a loose connection somewhere in the gauge or pump motor wires that tightens up when it gets warm, making a better connection. Does the FASS have an internal pressure bypass in case of a completely plugged fuel filter? maybe its building up pressure, popping off etc untill perhaps the filter ungells? Do you buy your fuel from a reputable station with high fuel turnover? It might be worth putting some new fuel filters on there, maybe theve done their job and filtered out a lot of crud from a dirty tank. Its happened plenty of times before.
All Im trying to say, with that long and scary list of potential issues, is that pressure spike and fluctuations are not normal in a fuel subsystem. Whether its very simple or very complex, id investigate further. Its more than likely a simple issue, but why gamble?
Attaching a mechanical fuel pressure gauge, even temporarily, would help a lot in diagnosing.
Edit: probably not gelling since the OP lives in Cali. Sometimes you just get in an East Coast winter state of mind!