I asked this question because the rules state that the top two players advance. Does first and second place have the same advantages when advancing to the next stage?

Yes, the first and second places advance to the next stage, but we are planning some privileges in the form of seeding for those who score 2 points immediately. It is difficult to say what results will be shown and how many groups there will be in which all participants score 1 point, so we will announce the scheme for advancing to the second round and the rules for the draw at the end of the first round.

And regarding the nickname S1VA - it is displayed like that in the tournament table... or is it from the letter "I"?

In the tournament table, it is s1va, and that's how it should be written. We have thousands of users on the forum, hundreds of players in Heroes 3, and there are very similar nicknames, so the easiest way is to copy the username from the forum profile, so there will definitely be no errors.