I'm late to this thread, read through it all and lived through today's ups and downs with Duncan - I was therefore so pleased to read your post to say all was well! Not toe curlingly embarrassing at all WWM2, just such a relief and lovely end to a worrying day for you.
We had a dog whose back legs would randomly collapse under her. We found that if we lifted her up, under her armpits so to speak, and held her up for a while with her back legs dangling, she would recover. The vet at the time said something about trapped nerves, drew diagrams for us to explain it all, but it was a long time ago, and I don't recall exactly the issue. He did tell us that what we were doing was exactly right and explained why holding her up for a minute or two worked.