Tiak

I don't know that all gems are doing the same thing, and I disagree with how you think the Shattering happened, but also:

We have lots of WoB about red eyes. The takeaway is that red signifies one shard coopting another shard's magic system and that, especially on Scadrial, it isn't necessarily connected to Odium. My suspicion is that people with red eyes have been spiked (with a foreign god-metal) and are being controlled by another shard manipulating Ruin's magic. I would speculate upon it being a yet-unseen shard, as I doubt Odium is going to be our only real villain going forward. Good villain shard candidates: Jealousy/Avarice, Indifference, Dispersion, Authority

Maybe the combination of Honor's powers with those of other shards creates blindspots in the possibility-tree? The three blindspots we see (Renarin, Lift, Dalinar) all seem to have a heavy influence from multiple shards

We've gotten a good bit of insights of shardic future-vision from the various holders and actions of Preservation/Ruin. One might speculate that Preservation in Mistborn era 1 was able to pull things off either because he was slightly better at seeing the future, slightly better at influencing unlikely outcomes in order for them to occur, or slightly better at making those outcomes seem unlikely until they do occur. If it is a matter of shardic intent then ruin and preservation are both much more future-looking in intent than passion/hatred-leaning-passion, and, for that matter, cultivation is also more future-looking than passion. It seems plausible that Odium is actually among the poorest shards at determining the future, despite seeing into the spiritual realm, though he may be the shard most motivated to try. Still, Odium was not able to foresee Dalinar's resistance to him, while Cultivation recognized it as a possible result of her work. Odium was not able to detect Lift in the vision. Odium was not able to see and expand upon Renarin's portion of the Diagram, when even Taravangian could (through cultivation). Odium actually seems to have quite a few flaws and blindspots in his vision of the future, though he seems more likely to use what he does see to influence people.

I doubt Navani will become a Radiant if only because Brandon will want to provide us non-Radiant characters involved in the main plot... On the other hand, Jasnah is about equally well-suited as Navani to bond the Sibling if not more so. She not only has knowledge of fabriels, but finding and learning the nature of Urithiru has basically been her life goal. She may already have a connection to the place forged from her searching. Brandon has essentially said that it is possible to bond to more than one spren, but there are limits, and it is complicated, and read and find out. This leads me to believe that it is a thing that is likely to happen over the course of the series. I'm currently guessing that Jasnah will bond the Sibling, though this may be an incomplete bond as she is divided between her bond with it and her bond with Ivory. https://wob.coppermind.net/events/156-words-of-radiance-lexington-signing/#e2841 https://wob.coppermind.net/events/221-words-of-radiance-omaha-signing/#e6202 https://wob.coppermind.net/events/259-oathbringer-leeds-signing/#e8735 https://wob.coppermind.net/events/218-words-of-radiance-chicago-signing/#e6647

The information we have implies that Parshmen were all transformed at once. We have no evidence of any sort of a piecemeal attempt to hunt down and mutilate individual Singers and cut out their gemhearts. Not only would that be horrible, but if the Radiants were capable of stopping them that way, why not just kill them all? Would they really be no survivors other than the Parshendi in dull form? Likewise, if that happened, where would the huge cache of fused have gone and why hasn't it come into play? It seems much much more likely that the Parshmen basically had the "connection" part of their spiritweb crippled so they could not bond spren or feel their connection to the world's songs. This would also go some way to explain how a bondsmith would be able to do this to all parshmen at once, as bondsmiths command grand powers over connection.

I have to wonder if their gemhearts might be a source of perfect gemstones. Perfect gemhearts could hypothetically be what allows them to capture and use stormlight so much more efficiently than other creatures.

So this theory has a good deal of explanatory value, but I think there's one thing that you're missing in it, namely that a lot of characters have this sort of intrusive thought, possibly from a cognitive shadow, namely,the words. Consider the words for a moment, for each radiant we know that these words spontaneously form in their head. We know that the spren themselves cannot tell the radiant the words, but else something seems to be in the heads of the radiants, telling them the words. Why can't this be the same thing that is giving Dalinar these thoughts? I would presume that it is the remnants of Honor/Tanavast. After all, Loras told us that he was dead long before we ever met him, but his will stayed active with his power. Maybe this is also at work with Honor's power. That could be why Odium has decided that he needs to do more than simply splinter Honor.