I'm not convinced (or dissuaded, depending).

Hard to see how a carrion-eater would evolve a body plan that raises his jaws high above the "big, dead plant eater corpses littering the landscape". I'd expect evolution to reward a four-legged stance for being as close as possible to where the carrion falls: On the ground.

And nose not on top, but at the front or downward-facing, to smell what's on the ground...I could go on and on.

Or if this really was a bipedal carrion eater, then I'd expect the front legs to not atrophy, but grow long and strong to haul the carrion way up in the air. Or pull apart carcasses and pull pieces into the mouth (poor T Rex's arms couldn't reach his mouth).

You did specify "big" carcasses, like the apatosaur, but that would be a pretty narrow specialization. T Rex would stand up on his hand legs to dig into a big carcass, I guess. The big jaws wouldn't be good for delicate work nibbling on smaller carcasses, but I guess would be OK on the big ones (almost as good as they'd be bringing down prey for a predator). Maybe. But OTOH, quadrupedalism seems like it would work for a wide range of carcass sizes. In the case of the big ones, they're not going anywhere, munch from the bottom and let it collapse into your gullet.

I'm not arguing utilitarianism because of some "Creator" having a goal to make "good" dinosaurs, but just evolutionary efficiency and fitness. If a posited carrion-eater is an awkward carrion-eater, I'd look for an ecological niche for which it's a better fit. T Rex was an apex whatever-it-was, and it didn't get that way by being sloppy.

The analogy with modern two-legged predators, that their prey is much smaller, seems a bit weak. For one thing, we're talking animals much smaller than a T Rex, and thus at risk for having their prey snatched by a bigger predator. Speed is of the essence for these smaller bipeds, they'd go for lots of small prey they can gulp down quickly and keep.

T Rex could probably afford to take his time to eat a big herbivore. And who says he couldn't sneak up on his prey? I'd say my posited ability to burst through foliage might've given him some element of surprise. And if he could burst through foliage, he could hide behind it, waiting for prey to come closer. Big doesn't mean he couldn't be quiet.

The forest verge habitat might have favored the ability to burst suddenly from hiding, and run down a plains-dweller that strayed too close to the forest. T Rex could've been an ambush predator many ways.

One last tiny point to gnaw on...the habitat might or might not have been a true "jungle", but if T Rex liked the taste of large herbivores, he wouldn't find those on a grassy plain, he'd find them where there was enough vegetation to feed the behemoths. Some place with trees for which those long necks evolved. Forest, jungle, tomato, tomatoe.

Anything but a carrion-eater.