Forbidden Journey is the outside seat. It doesn't matter whether left, or right. Gringott's is 6th and 3rd row. Also, for the Mummy, if it cannot be pulled down all the way in the back row, you can also move to the end of the row. As one time I was sitting with a couple where one of them had issues, they instructed them to switch, which resulted in the person having issues being at the end of the row, and finally they were locked in.
I also want to say that a lot of times people need help in order for things to get locked in. So, just because you do not make it once do not lose all hope. I feel the need to say this, because for example, I've rode FJ countless times and one time I was stopped and asked do to a test (which I've never done). All the woman simply did was had me pull down the bar and since I couldn't pull it far enough myself, I was out. However, I've since have been on the ride again. In most cases, they will give you a light nudge to make sure you're locked in. I'm sure some cases are impossible, but I'm just saying if someone doesn't even try to help you don't lose all hope. Besides, even if you can't make it, you'll get an express pass for one ride out of it. At least in FJ I did that one time.