I'll play avocado diaboli if you don't mind...
Interesting but somewhat unreaistic, seems to me. Wild rice is good, of course but you need several more millenia to have the locals get the idea of its domestification. Your initial supposition of such an early rice horticulture really looks ASBish. Why? Because until later times even after the arriva of the Europeans, Northamerican indians gathered rice growing in wild. They never raised it, or I am mistaken badly.
It is true that it was never raised in the wild, I just don't think it's ASB to suppose it couldn't have been. There were attempts by Indians to cultivate it (rolling it in balls of clay and dropping it in water), they came to nothing for a variety of reasons but mostly due to the ease in which the stands shattered. Wild rice agriculture in the 19th century suffered the identical problem, until a shatter-resistant cultiver was discovered. This has led, in OTL, to the expansion of wild rice agriculture. In my TL, a shatter-resistant cultiver is discovered at a much earlier point. In my opinion, it is unlikely set of events, but no less likely than the OTL history of maize. Mostly, it's that stuff that happened in OTL happens earlier, and is much more successful. On the basis of the crop itself and its potential I don't believe that it is ASB.
There was no populational pressure, even in Missisippan culture, and thus no need in domestification of rice. Especially when by 8th century AD there already had been in pace the main culture, zonated maiz. Rice is a very capricious cuture and is reasonably hard to raise. It will require communal works on irrigation and thus you will get societies close to early Chinese or eary Sumer.
Maize wa never developed from teosinte in this timeline. The Megalopotamians are not Mississippians, they diverged millenia earlier during the Archaic period. The early Chinese and early Sumer is roughly the model I am using for their development, and you're right, communal irrigation works are indeed required.
The most necessary task is to direct the initial waves of American colonisation to the region of Missisipi several millenia earlier simply to give the locas enough time to build civilisation here. The region in OTL was populated comaratively late because the waves of colonisation moved two ways - along Kordiliera down south and a much weaker one - to the east in Canada north from Great Plains. It were Great Plains ans deserts and semideserts to the south of them that prevented Stome Age colonists to get to the Missisipi fast en masse.
That's really the most promising region of all Americas but, as I said, it was colonised too late. And maiz, very sensitive to the length of the day, got there too late to let the agricultural civilizations in the region get on their feet and harden. When De Sotto got there, it was still in the epoch of early competing polices.
Jared Diamond, read him. His expanations look very solid.
That's an interesting idea, and makes sense. I may have to address something like that in the timeline, though I'll have to look more into the climate and geography of the early colonisation period. I am afraid that messing with that would stretch plausibility in terms of the Old World remaining uneffected though. Still, butterfly trap is useful. Perhaps things would be better if I'd included both wild rice agriculture without removing maize, but I liked the idea of removing Mesoamerican civilization and was afraid of opening myself up to accusations of Amerindian-wank.
My point is mainly that while I concede wild rice agriculture at such an early point is a low-probability event, I don't believe that it is of such low probability to require the introduction of outside influence. No aliens, divine interventions, shipwrecked Phoenicians, ISOTed agricultural equipment or magic is needed for the success of wild rice agriculture, so I object slightly to the label ASB.
That said, I always appreciate a good avocado diaboli.