Second question first.Yes and no. Doesn't that seat have a clasp of some sort to latch it down? If yes, then it is either a negative function or you are not engaging it correctly.If no, then you should install one.
First question. Highway speeds cause a considerable amount of air flow in and around the hull. This causes unsecured items to become airborne. Ever notice a boat cushion, cooler or life jacket on the side of the road heading to or from the boat ramp?