So.... You took guild wars and torch battles then combined them to make another mode that won't be fun but we will probably just do for the rewards? Well that makes sense, this is IGG we are talking about.
The time is a problem. The guild leader will select a time slot. Yet, no guild leader can select a time slot that will work for all 75 members. This will end up being as horrible as the design failure for torch battle. (For me, torch battle ends at 3:00 a.m. and all the action is at that time. Hate it.)
A better design would allow the player to select a time slot. Maybe 1/3rd of the guild should take each of the three time slots. That would also add some strategy to the game, deciding which players would go in each slot.