>So, sometime between 1956 and 1973 we, Americans at least, acknowledged that we were using "moot" to mean "of no significance in this discussion."

It looks like I must have been misunderstanding people for years then, i was only aware of the pre- 1956 definition (and i'm not that old!).

There are lots of Moot Halls in English towns. I assume that they were built for debating and contribute towards the use of the term "moot point".
Here's a couple:
http://www.btinternet.com/~lake.district/kes/kestic.htm
http://www.bedford.gov.uk/bedford/tic/john bunyan moot hall.htm
- put the words "moot hall" into Google to find others.