Whoa Dr Redhawk, this was a lot of typing! I hope it wasn't covered in your soil series, which I am slowly working through when I don't have enough else to read here (rarely). You answered a lot of questions I haven't even formulated yet.
All of the plants infected by Frankia, with one exception, are trees and shrubs, whereas among the legumes both annual herbs and trees may be infected by rhizobia.
What is that one exception? I have very little experience with Rhizobium, but a fair bit with Frankia, and I can't turn soil at my place without finding nodules. It's become a party trick for explaining nutrient cycling to visitors. But I wonder if I should encourage Rhizobium as well, come next spring. There are probably some on the many patches of wild & some cultivated clover that I have around.
This also makes me wonder about my soil pH, which I have never comprehensively tested. In "Teaming with Microbes" the authors assert that "nitrogen fixing bacteria generally require a pH above 7." But then "As soils become dominated by fungi, the populations of n-fixing bacteria required to convert ammonium into nitrates diminish because the pH is lowered by the acids the fungi produce." My soils seem to be both very fungal (just from looking at leaf litter and how quickly it myceliates, or how thoroughly flushes of, say, Sulfur Shelf or Dead Man's Fingers or Angel Wings dominate) and neutral/base. Due to historic logging, there are not a lot of conifers around to lower pH. Fungal prevalence vs. N-fixing bacteria sounds either/or and I'm sure it's more complex than that.
I also think there is an aerobic component to this process, that is, if oxygen availability drops below a certain point, it stops. But I can't find where I noted this in the books.
Because the reaction can only occur in a low oxygen environment, the process is often dependent on hemoglobin compounds found in the nodules, which are virtually identical to those found in the red blood cells of animals.
The plants provide the hemoglobin as well as the nodules?
Nodule size can be an indicator of which type of nitrogen fixing bacteria are present. (smaller normally means Rhizobium, large normally means actinomycete).
Having never seen a Rhizobium nodule, I had to Google them...I guess by "small" you mean "from the size of a brassica seed to a currant".
This is probably a good place to ask another question that's been bugging me: what is the best use for severed nodules? I've been burying them whole with whatever I'm planting. Maybe I should be chopping them in pieces, or putting them in a solution for dispersal, or both? Can Frankia populations be cultured a la compost tea? Would there even be any point to that in soil amply shot through with noduled roots? I don't know what happens to Frankia populations in a nodule when they are severed from a network.
"Teaming with Microbes" also says "some studies suggest that Frankia species can fix nitrogen without the association with a plant root," what?? Unfortunately they eschewed footnotes/endnotes.