I have been told, that when entering foreign country address, I may be asked to verify passport details, I am unsure how it is when i enter russian address. I guess that when entering invalid address, I may be unable to claim my domain in case my domain registar bankrupt. Thank you in advance.

https://www.hosting.co.uk/register-domain/su-extension/ says:

.SU registry asks that companies provide a copy of the registering company's Certificate of Registration or an excerpt from the Commercial Register. Individuals are requested to provide their ID card or passport number.

https://manage.resellerclub.com/kb/answer/1336#heading_5 says:

For Individual: Registrant's Name (preferably in Russian); Passport Information: This needs to include the Document number, Issued by, Issued Date of the Passport. Birth Date: This needs to be entered as a numeric value in DD.MM.YYYY format.

https://wiki.hexonet.net/wiki/RU says:

for natural persons: passport copy (personal details and issuance data)

Middle man style services:

https://xuid.ru/domains.php - "No Personal information required."