Peter, (and Julie some of this relates to your question too),
OK. Question 1: What kind of habitat do you want to create? If you want a deeply shaded understory, that's what this design will achieve. If you want much of anything useful growing under your
trees (there are a few useful shade-loving understory plants, but not many in relative terms--mostly woodland medicinals), then you need more space between the trees. Nuts and fruits are the last thing on a plant's energy-budget wish list, so if you want fruits or nuts in the understory, they need light, with a few exceptions.
Pawpaws will do OK with some shade, but, once they are past the young stage, they do much better with more light--more fruit production. Young trees HAVE to have shade, though . . .
I like your mix of trees in the overstory in terms of N-fixers. Mimosa can be a bit aggressive, but if you are driving by you'll be managing it I assume.
Apples have a LOT of pests, as do peaches. I usually suggest people go for Asian pears instead of apples. Much easier to manage pest pressure. There really isn't much of a substitute for peaches though, especially in MO. They do much better there than here! If you really want apples, then make sure you get one (or more) cultivars that resist as many of your
local pests and diseases as possible. My guess, though, is that you'll end up really appreciating your persimmon and pawpaws as time goes on.
You could have fewer N-fixers in the
canopy and fill your understory with N-fixers instead, thereby gaining more crop tree space, if you widen the spacing. You could also keep spacing as is, or maybe even tighten it a little, but open the canopy by chopping and dropping the N-fixers to keep them in check size-wise. With more open canopy and more crop trees, you would rely on shrub and herb N-fixers, and that could be dicey given that N-fixation requires full sun or close to it, but I think it would be worth a shot.
Beyond the canopy questions, the thing to do with understory is think through your access and management, then the pattern of patches under the trees. Make a bubble diagram showing the access and the patch patterns, then think thorugh the architecture of each patch--how high do you want vegetation and how dense in various layers, as well as what functions you want each patch to perform (e.g., soil improvement, beneficials habitat, weed control, food or other crops). Then you have the info you need to select species in earnest.
Don't forget that
mushrooms like shade!
Have fun!
d