1) The current version doesn't have the ability of associating a different thumbnail to the file. For example, you can't upload a .zip file and then show a thumbnail of a picture, instead by default you'll see a generic zip file icon.
2) The Gallery will currently receive files via the web page upload or via FTP but you'll still need to go the web page to bring the files into Gallery's DB after they've been upload by either method. For example, the FTP method is how I upload my wedding & vacation photos and then I go to the Gallery 'upload' page to bring them into Gallery in batch mode.
3) If you're familiar with PHP you may be able to hack it as such but that is not a functionality out of the box.
4) Normally all support is handled online but if you require anything out of the ordinary I'd suggest submitting a Support Ticket to see what can be worked out.
1) Wouldn't work because the thumbnails are based upon the file extension. If you leave the extension to be a zip file then the default .zip file icon will be displayed. If you change the extension to be .jpg then it will fail during the upload process because the code checks the file extension versus the file type (as identified by the PHP function) and rejects it if they don't match. You would have to hack around the extension check.
2) Again, it depends upon much PHP 'know-how' you have and/or are willing to hack the code.