Heard compass is not a bad school. Paying the whole year's fees upfront is indeed a challege and if you are not happy with the school, you'll probably need to be stuck there for a year to make your $$ worth it. Nevertheless looks like you do not have much of a choice if you are really moving here. It's true that siblings has priority over the newcomers. Major movements will happened during the summer hols & people may not want to come back so the other kid will have a higher chance to secure a place at that time.
Doha college has conducted their year 7 entrance axam for sept intake recently & my kids also have been called for assesment last month for sept intake. Looks like schools are already filling up for the coming sept . Since you still have 1 more week, suggest you call up the schools which you have submitted your apps & ask for status. If you don't hear any good news, I suggest you to take up compass to secure a place.