Oh, forgot one more piece of important info which I think supports the rest of the theory. One of the hoses to the filler neck, which has the blue diaphram looking thing on it, just connects back into the main large tube coming from inside car to the evap canister (which I'm suspecting is the tank vent). This means as fumes are venting from the tank, they can either go to the charcoal canister, or if the blue diaphram thingie is open & the valve on the filler neck is open, it can vent into the neck to allow gas to go in.
This video seems to confirm that the valve is activated by just inserting the gas pump nozzle, not electronically controlled. Also seems to confirm the one end (through blue thing) to the tank and the other end goes to charcoal canister (I didn't see that, but maybe it goes further down and connects, will need to recheck)
https://www.youtube.com/watch?v=zulI06TPBVc
And actually, the more I think about it.... if the vent to the gas filler cup while refueling is just there to let air out of the tank when gas is going in, and this is connected to the same hose coming out of the tank as the normal line to the canister, then when I vent that out to atomosphere, won't that "automatically" solve the venting while fueling issue? In other words, can I just get rid of the hoses & fuel filler neck valve altogether? I really wish there was more information on the web about this stuff from people who've done this before. It's like the one area of the boxster that doesn't seem to be heavily covered!
