It is not doable if you want to keep around all wordpress features. Possible casualties of such configuration are search and RSS feeds, probably also category and author pages.
If you go and actually create a new DB for each 1k posts then the game ends here as there is no simple way to share data between different databases. If you just want to create new tables every 1k posts then it will like multi site feature that require searching over the whole network, not elegant but possible.
As for the explicit points
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it is harder because of the artifcial number limit. Most schemes which distribute data between DBs that I know are based on some hashing of the url, but that reuires to have all the DBs setup from day 1 and not waiting to fill a quota before creting the next DB.
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It is if the owner of the site agrees to crippled functionally. For example it might be ok for a site which has only landing pages.
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yes, can be done but you might need to overide the db.php file