could you also discuss this point ("blogPost" and "forumPost" mapping to "post-weblog" and "post" instead of "webpage", respectively)?

That's a question only the Zotero developers can answer.

If fallback behavior is eliminated, updating update.xsl should be fairly trivial.

This might not be as trivial as I thought. There are some nasty corner cases, like
<if variable="editor translator" type="book" match="all">
You could replace "book" by all subtypes of "book", but you'd need to create a nested conditional because a conditional with match="all" will never test true if you are testing for multiple types, e.g.:
<choose>
<if type="book motion_picture graphic report bill
legal_case song motion_picture" match="any">
<choose>
<if variable="editor translator" match="all">
...
</if>
</choose>
</if>
</choose>

Plus it seems space delimited attribute values can't be easily changed with XSLT. Some hint at using regexp ( http://www.biglist.com/lists/lists.mulberrytech.com/xsl-list/archives/200907/msg00043.html ), but I haven't been able to get that working yet.

Just a quick update: I'm quite close to completing modifications to the XSLT 1.0 stylesheet so that it can convert CSL styles to cope with the elimination of fallback behavior. It'll take me a few more days to finish, but from the looks of it it shouldn't be a show stopper.