I agree that having the grab bag separate from the dinghy is not ideal but the weight of the bag is quite high and when you consider that the dinghy pack is capable of floating before inflation I didn't want to jeapardise that. Also my wife is not that strong and with lifting the bag from the floor behind the seat it would have hindered her to have to lift the grab bag along with it.
Some of the advice online suggested that for coastal waters the priorities were to exit the plane, board the raft and fire up the PLB. Frankly after that everything is a bonus.
I forgot to mention that like you, I had the PLB attached to my vest.