I'm sorry, but you're wrong about Farah. He did not go out with the leaders at London. He went out in 63:07, while they were out in 62:29; indeed, he was gaped early on, and by intent. Thus, while you say he "went for it," he didn't. But as for Rupp, if one were a betting man or woman, it would be rational to bet that he'll do fine and will make the team. However, as so many properly point out, the marathon is a vindictive mistress, and the only way one can tell how one will handle the marathon is by actually running it. So I have no idea how he'll do; but I'm glad he's running it because it will make an already interesting race more interesting still.
You have to look at the competitive landscape when comparing Rupp to someone like Tadese. Tadese was competing to win against the world's best; he was going out at 2:05-2:06 pace most the time hoping to be the last man standing. I'm sure he could have jogged a 2:08-2:09 with ease and no fear of blowing up. Rupp doesn't have to worry about being dragged along by a bunch of 2:05 guys. He can just relax in the pack and make his move late in the race, cruising in around 2:09 for the win. This will be nothing but a very tough long run.
I completely agree with this (and have been making this point in offline discussions for the past week+). Everyone holds up Mo as another example of Salazar not being able to coach the marathon and that track speed doesn't translate to 26.2, and I think they are completely missing the point. Mo ran in a hugely competitive race and went for it, "blowing up" and running 2:08. If Mo had been in a race with a bunch of 2:10+ guys and decided not to push the pace, he would have looked like a great marathoner. I think the same thing will happen with Galen. I think he wins the race, but for better or worse, I don't think it will give a ton of insight into how good/bad he will actually be in the marathon.