First off, there's definitely no reason why you couldn't use your mixpre with another recorder for live and nature recording. My thoughts on that for nature recording are that a rig would be harder to use with a separate recorder and preamp. One big advantage of the all-in-one unit would be less fussing with levels to make the recorder and preamp get in the sweet spot you want it to be in, especially with "in the wild" conditions where you have wind gusts etc to react to(I just went out 2 days ago in the Chicago blizzard and tried taping some thunder with heavy winds and spent way too much time trying to get the recorder and pre where it needed to be, not to mention in the bad weather messing with the LS-10 was a real pain). A 722(or 702 for that matter) is simple and effective in this regard, with one place to adjust your levels, not to mention that the recorder is bombproof for all terrain recording and built to be accessed from the front so it's very easily put into a harness that's manageable and weatherproof while climbing, hiking, whatever. One other thing I would say is that the A>D is probably better in the 702/722 than the Sony or the Fostex(I've never used a FR2LE so please forgive me if my assumption is incorrect on this). The preamp in the 722/702 is also a step up from the mixpre according to sound devices. I run a mixpre and like the character of it, I personally wouldn't say that the 7xx's necessarily sound better, I would say they sound different.
If you are comfortable running flash media and don't need a harddrive, you could easily get away with a 702 here and save some $$. The 722 is definitely a nice choice as well for the extra $$ and would take you far.
Hopefully this helps some.
J