Well, about the edit preview, don't worry about. Our Parser is complicated enough that anything like that isn't happening soon.
It already has: http://wikiwyg.org/wysi/
Yes, but for corner case stuff its output differs from MediaWiki. In other words, what you see is not always what you're going to get. For example:
* "JavaScript" renders as that on WikiWSY, but renders as "JavaScript" on MediaWiki.
* "&60;" renders on the Parser as "&60;", but on WikiWSY it doesn't show up.
* Don't get the ISBN / RFC / PubMed autolinking (e.g. "ISBN 1903")
* Nested links render differently - e.g. "[[a link[[b]]]]" renders like "[[a link<href>b</href>]]" on MediaWiki, but like "<href>a link</href>" on WikiWSY (drops the "b").
* a line with just "'''a" renders as <bold>a</bold> on MediaWiki, but as "a" (no bold) on WikiWSY.
* "{|{|\nx" renders as blank on WikiWSY, but shows an "x" on MediaWiki.
* "<pre>aaa</pre><nowiki>xxxx</nowiki>" renders as that string on WikiWSY, but shows as "aaa\nxxxxx" on MediaWiki.
But I do think WikiWSY is impressive. Getting something client-side that matches the server-side Parser for core functionality is very hard. Getting something that's "quirk-compatible" for the less common stuff is very very hard (and would probably require literally porting Parser.php from PHP to JavaScript; and then keeping it updated as Parser.php changed - lots & lots & lots of work; it could even be easier to write a custom PHP-to-JavaScript "compiler" to do it automatically).
All the best, Nick.