Tournaments and player rating.

Let's imagine player B sits down to play against player C and wins 10-0, but is too lazy to post.

There's no way to regulate this. What can you do if players agree to play a casual game without reporting it? You can't force them to post a report.

The second option is that they simply don't play against each other during the season. So what does such a rating reflect?

Such a rating reflects the current state of affairs. If player B has the potential to beat player C with a score of 10:0, it means that their rating is currently too low. If both players play a sufficient number of games against opponents of different levels (not just 1300 and below), player B will significantly increase their rating. I believe that 100 games are enough to establish a roughly accurate rating for a player.