An enumerated example is not a list item. I don’t think this counts as a strong argument against (3). I did assume a minimum of two list items would only be expected in lists that are not preceded by a blank line. If this makes it to complicated, I’m fine with doing this idea.
The reasoning behind (4) is that a tight list could well be a child of a paragraph (in output formats which support this nesting), whereas a loose list, which can contain paragraphs (i.e. blank lines) itself, seems strange inside a paraphrasing and thus could only end it.
<p><list.tight/></p>
<p/><list.loose/><p/>
Anyhow, I prefer (2), too. The colon at the end of the line preceding a list works in two ways:
- Existing content in many languages will have a colon introduce a list without an intervening blank line. It works as a heuristic rule.
- New content in any language can be authored with the colon as a new active markup character.
The problem is that for much of variant 1 the colon should be retained in the output, whereas it should be dropped for many cases of variant 2.
This can be done with an additional rule, but I’m not sure whether that would still be acceptable.