There's a relevant article from the Columbia Journalism Review entitled Shhh! It doesn't matter — A "moot" discussion.
“Moot” is an old legal term. It originated in the twelfth century and
meant either “A meeting, an assembly of people, esp. one for judicial
or legislative purposes,” or the place the meeting was held, according
to The Oxford English Dictionary. It evolved to mean an argument, then
In fact, in Britain, “moot” can still mean “arguable” or “debatable,”
exactly the opposite of its American usage.
There's the OP's first definition: Subject to debate.
“Moot court” is so called because points are debated in them, not
because the points debated are, well, pointless to debate.
There's the OP's second definition: Of no practical importance; irrelevant.
Somewhere around 1900, though, the fact that the “moot court” cases
were hypothetical, or not worth arguing, began to overtake the
traditional meaning of “moot” as arguable, according to Garner’s
Modern American Usage. Some held out for the traditional meaning; as
recently as 1980, John Bremner wrote of “moot” in Words on Words: “It
is mistakenly used to signify that something is beyond argument, that
there is no point in arguing the question, except in the technical
legal sense that something moot is something previously decided.”
Moot court continues today, in which law students vigorously debate an issue which is of no practical importance (because the issue has already been decided in a real court and the moot court wouldn't have any effect on the existing ruling).
(While the purpose of the article was to correct the incorrect usage of mute point, the etymology and evolution of the word moot was still quite useful.)