To weigh in on the proposals:
(1) (single-digit start numbers) seems okay to me. It might fix some
practical problems (and might cause some others). On the other hand,
even with this fix we’d have a mismatch between parser behavior and the
spec of the sort described in commonmark/cmark#204.
(3) is bad for the reason I just gave in my previous post.
One-item lists are, in general, useful, so we don’t want to
rule them out altogether. I suppose we could require at least
two items when the list interrupts a paragraph, but that creates
difficulties for parsing: you can’t know you’ve got a list until
you’ve parsed the whole list item and seen what comes after it.
(4) has a similar issue: we can’t tell if the list is going
to be loose unless we’ve parsed the whole thing. And I’m not
sure what is gained by allowing only tight lists to interrupt
a paragraph.
(5) is going to be problematic for people who wrap their text
to a reasonable width (say, 72 characters), and also for people
who don’t hard-wrap at all. And I echo @mity’s worry about surprising
behavior.
(2) seems the most promising to me, but there is the worry
about languages with different punctuation conventions.
I guess it might be a good idea at this point if someone
summarized clearly and concisely why we need to change things.
My own intervention above was motivated by
https://github.com/commonmark/cmark/issues/204, but I believe
that issue could be handled at the implementation level without
a change in the spec. At any rate, I’ve written a Haskell
implementation, roughly following the same strategy as cmark,
which gives the right results in this case.