1) Cannot be exactly defined because it uses generic detection which covers multiple viruses or entire virus family,but it covers around 36.000 unique viruses.
2) Currently not available,but it has very good generic detection and very fast antivirus team response,so they can cover this little "hole".
You might expect this feature in some next version.
Heuristic scan is already implimented for Internet Mail,Outlook/Exchange and The Bat! mail scanning modules.
3) You can adjust more settings for mail protection by editing avast4.ini file located inside C:\Program Files\Alwil Software\Avast4\DATA\ folder (this is default installer path)
4) There is currently no Russian language interface for avast!
Are you interested in translation?