The curious thing here (your very detailed report is most helpful) is the expansion of the FW tank and the rush of air when the filler cap is opened. This sounds to me like a breather issue. There must be a breather to let air into the tank when water is pumped out. Is yours blocked or even removed.
I have a 2003 B32, not 36, and I have to note that I am not sure where the breather is. There must be one though.
You probably have the same pump as me. They are noisy. When I had fresh water in the bilge there were 2 issues: minor leaking from the pump itself (cured by stripping down and refitting with gasket sealer) and leaks from one of the pipe junctions near the pump.
The pump has a strainer which can be unscrewed and cleaned out. Mine was clear after 18 years’ use.
Hope this helps.