typical fuel gauge/instrument cluster question

Oh, no doubt - same here. I'm just trying to get a feel for whether a faulty IVR could be at fault here. I'm probably just going to find a trim pot and check its behavior on the bench with varying resistance. That said, if it doesn't respond with resistance, is it safe to assume that there wouldn't be much harm in installing a solid state regulator without disassembling the fuel gauge to remove the old mechanical?

The two regulators wont work together. The original mechanical limiter will need to be disabled. I completely remove them from those gauges I service.