...then the following situation is possible:
A player creates several tournaments at once (let's say 10) and gathers a quorum consisting of his friends (let's say, representatives of one Clan). Other users simply don't have time to physically play in all these tournaments, and therefore the player and his team earn a lot of rating and Grails.

There is one excellent way to avoid this headache, and it is to solve problems as they arise, rather than in advance... Of course, foresight won't hurt, but it's not always appropriate...

Here is a simple argument in favor of this idea: if only representatives of one Clan participate in the tournament, then no one else simply knew about this tournament - it is an intra-clan tournament, and we do not consider it at all...

If the tournament is public, like your current tournament, try to organize it so that no player other than the members of your Clan registers for it... Will you refuse them registration? That is, it is impossible, even if they really want to, to hold a public tournament for players of one Clan... Even if they all conspire and deliberately go for fraud.

And what does it mean that "Other users simply don't have time to physically play in all these tournaments"? Some managed, and the others didn't? And what are ten tournaments at the same time? If they plan this scheme, they will not see high places - it is clear that it is impossible to play many tournaments at the same time and achieve impressive results everywhere... Are we not talking about a "cooperative" here?

In addition, increasing the number of unofficial tournaments reduces the average number of participants per tournament, thereby depriving it of a quorum. This is simply a doomed undertaking.

That is, the problem is at least only theoretical...

This does not mean that they will never be able to implement such a situation, but let's wait for at least one precedent, which will be the basis for revising the idea. And for now, we can quite leave this option...