do i understand this correctly: the trust already has purchased those coins and now some deep pockets can invest into bitoin? if they sell the shares quick, they might buy more bitcoins ?
Yes, if they sell out they have to buy more bitcoins.
is it possible that this fund was the whale that drove price up in july/august ?
No, they have only bought 17,800 BTC so far. But people who knew that this was going to happen can have bought large amounts.
17,800 BTC is nothing. They will start to buy more soon