Mark,
At this point in time, I'd consider getting hold of the dealer you bought the tractor from first, then the back hoe manufacturer.
Explain the problem to them and get their advice. Also, tell them what you've done already and what you may be planning (that is, the gear puller or the allthread trick).
In one of the posts, you mentioned this was the first time you dropped the BH off. That adapter may have been stuck on long before you bought your tractor.
Since you've tried a prybar, I don't think a puller is going to help either. Personally, I think you'll end up cutting the adapter off with a grinder.
Since the adapter does wiggle, it's unlikely the cause is rust. A burr may well be the issue (with precision machined parts, it doesn't take much of a burr either), but since your prybar actually started bending during your attempts, that would have to be one heck of a burr...of course, I'm assuming you didn't "cock" the adapter when prying and you did pry from differing areas of the adapter.
It's possible this adapter was buggered up before you bought the machine. You just didn't notice it until recently.
When you contact the backhoe manufacturer, you may want to ask about replacements adapters, since it is possible you'll end up destroying it to remove it.
GOOD LUCK!