However, it's currently at 139,000 actuations and Canon rates the "life expectancy" for the 5DII at 150,000 so I'm debating on whether or not it would be worth the upgrade to either the 5D Mark III or the 1D X (or if my money would be better spent on new lenses instead)
Welcome aboard. From what I've heard/read, that number is an 'average rate of failure', not a life expectancy. It might last another 100,000 clicks. And even so, the shutter is a serviceable and replaceable part. You could probably get a new one for $250 (don't quote me on that).
regarding nature, landscape and wildlife photography, if shooting the same subjects with the 5D Mark II, 5D Mark III and 1D X, would there be a significant or noticeable difference in the quality of the shots taken with the three (assuming all three photographed the same thing)?
Hard to say. The 5DmkIII just came out and the 1Dx isn't out yet (as far as I know). If you're shooting at lower ISO, the difference probably won't be easily noticeable (or maybe not at all), but at higher ISO, the newer models will likely show less noise. If you let that transfer to faster shutter speeds, then you are likely to see a difference in terms of the amount of camera shake blur.
There's usually a LOT of motion at the smaller gigs so I have to shoot at a fast shutter speed to avoid blur and I either end up with dark, underexposed shots that require a lot of post-processing work or I crank the ISO and end up with extremely noisy pictures (with lots of hot/stuck pixels to manually edit out).
Underexposing and trying to fix in post, is almost always going to be worse than just raising the ISO in the first place. There are plenty of great ways to reduce noise and if there is any consistency to your photos, the process can be largely automatic. As for hot/stuck pixels, if they are consistent, the process of fixing them can be automated as well.
would I see a drastic improvement in this respect if I upgraded to either the 5D Mark III or the 1D X?
Well, they are saying that the level of noise, at high ISO is less with those cameras, so if that is a major concern of yours...then you might see a lot of improvement. Will it be 'drastic'...I can't say.
Meanwhile, I guess I'm not sure why the 5D Mark III is still being touted as an "advanced amateur" body whereas the 1D X is for "real professionals." Fourth question: in what scenarios would the 1D X be considered a necessity over the 5D Mark III (or at least be considered a very worthy investment/worth the extra $3,300 in cost)?
The difference is the 'level' of the body. The 1Dx is a pro level body. It has all of Canon's best technology and it's made to be a tool that professionals can count on. If you haven't compared a pro body to a lower level camera, I'd suggest you get into a store and see for yourself. These pro bodies are build like tanks and can shoot in a pouring rain.
in what scenarios (if any) would it be a necessity (or at least very worthwhile) to purchase the 5D Mark III as a replacement for the 5D Mark II?
If you want the ability to shoot at higher ISO levels, with less noise, then the upgrade might be worth it. If you need duel card slots (more important for some than others). If you find that the AF system in the mkII, just isn't up to where you need it to be. If you just want to be the kid on the block with the newest, bestest camera...:er:.