In short, not many games have been played, this scheme has exhausted itself in this form, and the format will need to be revised.

I've been saying this for a long time. Although initially, the "favorites tournament" was created specifically so that players could play with their favorite Castle, with such a number of participants and the number of participants in the group, it doesn't really make sense, because people very quickly lose motivation and are not in a hurry to finish losing games, giving priority to other tournaments.
And 4 out of 7 people leaving the group is not serious. IMHO.

For now, the group stage is extended by 2 weeks to 03/29/16.

I don't understand, and why? I think you can safely close the groups. Why drag the latecomers by the ears, who will still drag it out until the last moment? You can schedule ODD (one-on-one duel) for the important matches that decide something, and that's enough, and deduct points from the rest, noting it in their personal file. IMHO.