One additional discovery is that TEA+hydroxide solution will readily reduce ferrous sulfate to metallic iron, as confirmed by using a magnet to confirm it is iron and not ferric hydroxide which fell out of solution. This does NOT happen on addition of ferric sulfate however. I do not have a good way to test if this happens with ferrous oxalate (only have ferric).
I found one paper which discussed TEA+silver nitrate reaction, but it was fairly unclear. It seems TEA will first complex with silver nitrate, then it will form silver metal and throw off the nitrate, and then with enough time eventually form silver oxide. However, this does not explain the formation of brown precipitate followed by it going into solution.
One interesting test I did was adding benzotriazole to the developer solution. It resulted in immediate precipitate of silver. Upon adding thiosulfate, it all went back into solution. I wasn't aware benzotriazole would cause a reaction like this. Paper development using the thiosulfate containing solution did actually work, but the thiosulfate was faster at bleaching the image than the TEA at complexing with ferric salts, so the trade off is that you go quick and either get orange whites from ferric hydroxide/salts or go slow and lose density in shadows and get an overall grainy appearance.
I also how pH level works, and seems that hydroxide is not actually ideal for this but some alkali is definitely needed. At neutral TEA ph (~10) the solution goes off very quickly due to ferric solubility limits, but also gave much less risk of solarization. At hydroxide pH (14), the solution lasts much longer but much more likely to give solarization. At carbonate pH (~12), solarization was slightly less but definitely still there. One interesting note however is that solarization can potentially be written off as a "wet and untoned" phenomena. The appearance of solarization becomes much less obvious when dry and nearly invisible when toned with a gold toner. However, I also tried a few other hail mary chemical additions to increase dmax and lower solarization risk. One really interesting addition is PEG-3350 (easily available as MiraLAX in the USA). PEG in general is talked about in some of those silver + TEA nanoparticle papers, so decided to see what would happen. A small addition resulted in slight speed decrease, slight increase in contrast (clipping highlights) and significant decrease in in solarization and increase in dmax. However, a larger addition of PEG caused these benefits to be eliminated and instead was left with increased solarization and decrease in overall contrast. PEG is a pretty weird chemical that moderates how silver salts are reduced and seems to moderate only "surface" and not "chemical" development (as observed and documented in lith developers), however it seems little is understood about how the chemistry around that really works.
I also tried using the TEA solution without fixer, clearing in a more dilute TEA+hydroxide solution, and then rinsing for a while. This resulted in good looking prints, but left silver salts that seemed to not be light sensitive behind as confirmed by putting into an ascorbic acid developer. Exposure by UV lamp did not result in any printing out of the image. I'm unsure fixing can be safely skipped or not due to this effect.
Also one other important factor is that the TEA developer initially will produce a very deep contrasty image, but as development progresses orange/red solarization takes out the deep black tones and additional highlight density comes in. This is in contrast to other non-reductive kallitype developers like citrate, where the full image is "done" within seconds. Leaving the print for 5m vs 2.5m resutls in noticeable decrease in dmax. This seems to indicate the TEA will eat away at the silver metal that develops, but despite TEA often being called a silver solvent, I've not been able to confirm this in anyway. A piece of dmax developed/fixed film resulted in no evident density change over ~20m in the solution. This could alternatively be the brown precipitate seen with silver nitrate+TEA going into solution slowly, but that just makes me question even more what that brown precipitate actually is, and if it's safe to keep in the image if there were some way to selectively make it insoluble
Also the non-ideal is likely buffered. It will fog with any developer tested. However, I've also gotten great results from a known to be buffered paper in this, with perfect white highlights. I only have two papers really that give good results to mess around with (more on order). One is Arches watercolor HP and the other is H(some german name) platinum rag. watercolor is known to be buffered and "unsuitable" for kallitype/platinum but works better than the platinum rag for me in general (using TEA, TEA-acetate, or oxalate), giving deeper dmax and cleaner whites. I suspect the non-ideal paper may contain small amounts of salt such as sodium chloride which would definitely produce fog in the TEA developer at least