A couple of options here. If you don't want to save the existing regulator then just wrap it a little, put it in a vise (if you have one) don't worry about the gauges or anything and find what you can to untwist the bottle.
We can only make a guess that the previous owner didn't use locktite
On my first one I wrapped a thick towel around the regulator, placed it in the vise, tightened it up and made sure the gauge and other parts weren't getting damaged and placed a strap wrench on the tank to get it off.
The second one I did came off much easier.
No need to take the screw out of the regulator body, that only takes off the top for when you want to access the spring and discs inside, it won't help to get the regulator off the bottle
No need to worry, be absolutely sure it is 100% empty and it will wind up coming off at some point.