Well, hell and damnation and similar.

The TV is now hung, the holding, even with two-and-a-bit bolts rather than
three, seems more than adequate, but...

the hinged-arm support doesn't hold the set truly horizontal.

In fact it holds it nothing like horizontal. Even with the hinge-adjustment
bolts tightened almost to the point of locking the whole thing up solid, the
set is still at an angle, both with the arms extended and even in the
against-the-wall position. There's no way I can live with it like that, so
it's back to the drawing board (or possibly the TV-stand-on-casters even
though that will take up a lot more room than I'd like).

Thanks to all for the thoughts and advice.