After you get the pressure down, maybe you can unscrew the valve and spoon out the debris. Maybe you can suck some out with a tube taped to your wet-dry vacuum. And as far as getting air in there, I expect you could blow air in through the vent. However I would want to get the debris. Another idea is to blow air in to hold you over while you take steps to remove that tank and going with a common pressure tank. Just don't blow more than 60 PSI.
These are just ideas, and I have never actually done any of them. Some may be bad ideas.