Wow. Lots of great info and ideas here. Let me address a few points that have been raised.
I don't mean to oversell the slopes here, but I will not drive my L on a side slope in several sections. Maybe my Pucker-o-meter is more sensitive than some, but I really don't want to lay down a brand new cabbed tractor... Also, I do not have a photo of the worst section. My excavator guy could not work it even with his tracked skid steer, so he graded it with his excavator bucket from above (he's an artist with that thing). I don't think it is mowable at all, unless i got a sickle bar that could reach down from above.
Using the L- The mowable sections are mostly pretty short (except the run down to the street), so that is like using a sledgehammer to kill a fly. I do have wheels set at widest now, also due to the slopes. It helps, but there is still pucker at various points. I also have R4s and they are filled with rimguard, so it is pretty heavy - also not the best choice for finish mowing. It is set up more for logging work, grading, plowing, etc. I also suspect that running up/down with a RFM or the like would be painfully slow, as that would be the only option for the smaller steep sections. Those sections in the later photos are only 20-50' wide, so that is the length of the run for an up/down mowing. That would mean back down, lower mower, cut going up, raise mower, shift tractor over 5', back down, repeat... Very inefficient. And I don't have an RFM, so I still would need to buy something, no matter what. If I had one, I'd try it and see...
A $10k+ mower/tractor is NOT in the budget at this point, no way. So new x700s, BXs, commercial anythings, or other such options are not in the running. Even used most of these things are too pricey if functional, and I really do NOT have time for a "project" right now to even bother to find a cheap one...if I could. Plus the amount of mowing is so small, these solutions also look like the sledgehammer/fly thing.
From everything I have read and seen, ZTRs sound like the wrong solution for this. For one, not many obstacles to need the turning ability. Secondly, I hear that they slide a lot if it is damp at all. Several posters in this thread said the same.
I may need something that might just be a stop gap solution until I can figure out what it really requires. Preferably used. I have a friend with an X320 that is a year old. I am going to have him bring it out and see how that works as a test. That should let me know if a 2wd decent mower is up to the task. If it is, then this may be the solution for now and maybe for good. But that will be a week or more until that happens.
Some individual points:
The pics are not current - they are from right after it was graded and covered for erosion (last year). But the grade is not going to change much, if any, as it is mostly dictated by the elevations/terrain, so the suggestions to regrade are not in the cards. It was a bit of a challenge to work with the lay of the land, but I did the best we could.
Schnauzer - That gets a bit steeper than what I have, but the left side is in the zone. Always tough gauging these sorts of things on photos...
JD420 - curious as to why you say the get a 4wd, but then say your 420 2wd will do it? Is there something special about the 400 series you have?
Tractorshopper - I think I am a bit steeper than that in the parts I am concerned about. And there is no flat spot at the bottom like you have, so no way out if it starts to go bad. If I tip it on the side there, I am into the woods...