I also think although I could be wrong but I don't think there is no commissions at least up to a certain level. However dont know if I am charged more on another side.
Network fees, it is an interactive trade to the blockchain. I have no idea how their great offer works (haven't used dydx yet), they just offer no fees after "depositing" to L2 and fast trade execution which both apply if funds are pooled in one wallet. I think they may still control your funds at some point (cmiiw).