Putting your arm on the seat back in front isn't going to make the slightest bit of difference. They arn't designed to take any load in that direction in fact most will just rotate forward.
And its not the done thing in any of the operations I have seen (but always refuse to get on) to put life jackets on before departure over water. It would cost a fortune. And within a week only 50% of them would still be working anyway.