In my opinion:
Numbers 1 - 3, 5 & 8: Show the search results of the Wiki you're currently on, and then a seperate column besides it for results from other Wikiprojects (in a list), entitled like Results from other Wikiprojects or something like that. This means that the results of other Wikis can be found, instead of being somewhere at the bottom where most people don't look. Don't mix the additional results with local Wiki results, since it could look (in my opinion) very messy when that is the case. For example: English results mixed with languages that don't use Latin letters (like Arabic) and vice versa.
Number 4: It would be useful to provide an option to turn it off, in either the Preferences page or when viewing the search results (or maybe both?). In my opinion it is better to avoid using prefixes, especially when someone doesn't know about that.
Number 6: Images (the first image that is used on the page) would be useful, so that the (possible) reader globally knows what the subject of the page is. Kind of what currently happens when searching on the mobile Wikipedia site.
Number 9: I think showing the most relevant 1 to 3 results at the top, and then at the bottom of those results the option Show more results (hot-loaded (not loaded when loading the search page for the first time (or something like that)), so that searching stays as good as fast as it was before the implementation.
Number 10: I don't even know why you would look through 10,000 pages to find a subject... I think capping it at about 500 to 1,000 should be more than enough, since the testing shows that generally only the first 3 links are being used.
BTW, this message may contain some Dunglish