I have idea about how sitting in general can be solved, not only for this tournament.
You should implement "vacation mode" for X days per year (let's say 60~90 days per year per player would be reasonable). Then, every player can turn on and off his "vacation mode" by writing start period (day/hour) and end period. In that timeframe, all his games should be frozen, so nor him nor his opponent would be able to play a game. Minimum duration of the mode should be 24 hours from the time when mode is turned on. Bad side of this is that opponent who is not in vacation would have frozen games here and there, but there are many positive sides of this suggestion, including that sitters would practically not be needed anymore and all turns would be played by account owners.
Also, I think there is an loophole in rules:
No tiebreaker games will be played. Ties are broken as such:
Head to Head games played throughout the tournament
Number of Wins going second
Seed at start of Round
Player Score at Time of sign up
Looking at how the system of the tournament is made, I suppose in every or almost every round there will be some tie breakers, so this rule is very important. However, the underlined first tie breaker rule is easy applicable when there is tie between two persons. However, in this tournament format, there will much likely always be tie between 3 or more persons, and in that case we would not have equal amount of games played among tied players throughout the whole tournament. For example, if player A faced player B 2 times, and player C 3 times, but player B faced player C only one time, how will then be applied the tie breaker rule? Player A has 5 games played, player B has 3 games played, and player C has 4 games played, so their head-to-head statistics will not be comparable. And do not forget, ties will happen in every or almost every round, and this is the first tie breaker rule, so this is one very important question to solve in advance.