"A pipe wrench should do the trick while you've got it drained to free
the plug... "

Or use a friends vice. Or just go to HD and buy another longer nipple
and cap.

This sounds like the classic problem of the idiots who tighten wheel
lugs with an air wrench so tight that you can't get them off with the
usual wrench provided with the spare. A friend of mine had a place
tighten his that way. He discovered it right away and went back. The
service guy tells him that they tighten them to the appropriate ft-lbs
of torque. To prove it, he sets a torque wrench to that number, puts
it on a lug and when it gives at the set number, says "See, it's
right!" The imbecile not realizing that all he proved was that it was
at LEAST the set torque. It actually was torqued to far more.