Well, after an hour and a half, much swearing, removal of paneling behind the valve, and then finding out the valve originally there isn't even manufactured anymore (something turning out to be typical of my beloved shanty); I have a fabulous new shut-off valve and it was only $90. (Why, yes, that IS a run-on sentence... and sometimes such sacrifices must be made for the sake of sounding conversational, n'est pas?) Not only am I singing my plumber's name to the hills, I got to observe and learn what to watch out for when I eventually attempt such a repair myself.
I know I could have done it myself now... and it would have been four hours, much swearing, and probably a small stroke from boilingly-high blood pressure. I know my limitations. This is one of 'em.
Now, on to replacing that shut-off valve... if you hear me ranting just ignore it, mmmkay?