I replaced the drivers side seal today. The factory seal was pretty ratty but here is a picture:Surprisingly, the factory seal looked more like the Fairchild part, but the proof is in the pudding: the DC part fits better and doesn't hinder the...
Most modern gaskets are silicone similar to that. I don't know what they withstand as far as tightening goes. You would want to be aware that you can overtighten them and damage them. Modern gaskets are also either in channels that keep them...
It will compress in time. If the body lines are flush, it is adjusted proper. Also need to be sure the plastic on the door latch pin is in good shape. If it is worn, it can cause it to be harder to latch as well.
Seem to recall that there is a check valve in the fuel line but could be wrong on that. Either way, it is definitely a case of the check valve for the front tank allowing it to go into the return side despite being the rear tank in operation.