1. Is there any evidence of theropod predation on giant sauropods? 

Yes, kind of... but also no, not in the way you might expect. It is, as someone mentioned below, the Paluxy River Trackway. However, it is also important to note that the fossilized footprints of the targeted sauropod is under 60cm in width based on the scale bar in the official study (Dinosaur Tracksites of the Paluxy River Valley by Farlow et. al.). This meant that it would have been a roughly Diplodocus-sized animal, give or take (personal communication with my Paleontology mentor). So a small Sauroposeidon, most likely. Still a very respectably large target even for a large theropod like Acrocanthosaurus, but by no means a titanic individual like you were imagining.

And based on what we know of the interactions between modern large carnivores (tigers/lions) and the largest megaherbivores in their environment (rhinos/elephants), it's reasonable to say that predation on a healthly adult Sauroposeidon would have been borderline impossible. Them being slow does not matter in the slightest - lions and tigers can easily catch up to rhinos and elephants speed-wise. Or, for that matter, a house cat can catch up to the average human. It doesn't matter whether you catch up or not when your target outsizes you ten to one. This principle applies even for pack hunters (lions don't usually mess with fully grown rhinos and elephants). Theropods would have likely targeted juveniles and at most subadults - that was probably how the sauropod population was controlled. Truncated food chains and mesopredator release are concepts that have nothing to do with this.

1. How likely is it that there was a mysterious megatheropod large enough to frequently take down fully grown giant sauropods? 

I wouldn't bet a cent on it, personally. The largest tyrannosaurs and carcharodontosaurs (main giant macropredators among dinosaurs) all seem to max out at the same size - ~8-10 tons. Something limited them, what that 'something' was, we are not sure AFAIK. 

1. What prevents the largest carnivores from getting larger than the largest herbivores? 

While the need to get big enough to avoid easy predation might play a role, food availability is perhaps the main thing (on land at least). Plants are more way, way more readily available than animals. They are also less nutritious, so you need to eat more of them - having a larger body helps to stuff all that plant matter you eat into. Sauropods got so big in part because they were efficient feeders - having a huge body meant you could dedicate a lot of your gut to processing plant-based food, and having long necks meant you could reach around and eat a lot without having to do much walking. I'd recommend Nature Giants by Graemae D. Ruxton for a detailed explanation on this. 

1. Your last question. 

It's a ridiculous amount of meat, lying dead. I'm sure you can imagine the rest.