There is no way a valve could be threaded on completely and be cross threaded. The primary reason I bought my first compressor years ago was to avoid the extra rip off charges that the local dive shops were extorting from air gunners for annual visual inspection fees. If they are filling your tank the air should be dry. Annual visual inspections in addition to the 5 year hydro testing is extortion in my opinion. I have a local friend and member of this forum had a dive shop "hydro test" his tank and instead of an inspection sticker they labeled the tank with a Sharpie. But that's another story. They went out of business, thank goodness.
If this tank had been mine I would have removed the valve prior to it's first inspection. Sometimes tanks from reputable vendors arrive over tightened. My suggestion to remove a stuck valve is to use a strap wrench to hold the tank and to use a large rubber mallet to hit the side of the valve a time or two to bust it loose. Don't hit the valve on the handle! You could also assist the process by partially filling it to 500 psi then open the valve fully without the fill hose. Cooling the valve will create a temperature differential to the tank it's attached to so it will loosen much easier. It sounds like his testing facility is after easy money and didn't put much effort into removing the valve. If a rubber mallet could damage the valve it would have to be made out of pot metal and would be too dangerous to own in the first place.
You probably didn't have a defective tank or valve. It required elbow grease. Buy a compressor. The time savings, convenience, and avoiding this type of treatment by a vendor is all a bonus in addition to the fees you'll save.
