You could try to pop the valve open by running a thin wire, like a coat hangar down the fuel tank filler neck. I would loop the end so I don’t accidentally punch a hole though a rubber flex tube.
Removing the rollover valve would require the tank to be dropped to access it. It is located where the filler neck attaches to the tank itself, and not accessible with the tank in place. I would advise against removing it, as it is a simple safety feature that could save your life. Simply freeing it up and opening it is all you need to do.
As far as the small leak, a loose or damaged gas cap is only one possible problem. A small leak can happen anywhere in the evap system, from the engine vacuum source to the charcoal canister, purge valve, and gas tank itself. These can be hard to locate, and a smoke test is usually used after the gas cap is checked and eliminated as a problem.