I had an alternator do that to me once... It's definitely a problem with the internal voltage regulator. And the easiest way to fix that is to replace the alternator entirely.
For what it's worth, I drove home for about 45 minutes after the alternator started doing this... It was about 4 years ago, but I don't remember there being any ill effects. Perhaps the battery died a few weeks later, but who knows?
And as for how to work on the upper alt bolt, I first start by using a crowbar to pry the alternator out, holding it with my left hand while I adjust the nut on the tensioner guide, at which point the alternator shouldn't move much. Then I use a ratchet with a 3" extension and whichever socket it is, and tighten up the upper bolt. It's a close fit, but it works. Hey, maybe a cool side effect of having an electric fan is also getting more room to tighten the alt bolts!
