>1. Does search act differently depending on which directory my site is in?
This question is either about DMOZ search (in which case the answer is irrelevant--hardly anyone uses it) or it's about some real search engine (in which case ODP editors probably aren't in a position to know the answer -- and if they did know, they wouldn't know when it would change.)
2. If site is added to russian directory, is it possible to be added to english version later?
The ODP's practice is to consider a site for listing in ever applicable language. In each language, the site is judged ONLY on the content in that language, not at all on the content or listings for other languages.
3. If main url is added, is it possible to add sub-urls (say,
http://www.rususa.com/fun/radio as resource for live radio feeds) to DMOZ later?
Sub-URLs are "the exception" and you should not expect to get them--most sites don't, and we couldn't build a directory without processes that assured most sites didn't. But ... it is possible.