Considering their treatment of Sheepstealer and Dreamfyre, I serious doubt we'll get anything with Cannibal.
I would agree if Condal hadn't said anything about "new dragons" next season. That could be limited to The Dragon Pit dragons perhaps, but I feel like it won't be.
Isn't that the core issue though, such knowledge wouldn't be something the majority of people would know, at best only the other highborns and even then not beyond a few generations? Plus the whole genetics thing isn't even the case in-universe for the other houses as far as we know, at least in the cases of the Starks and Targaryens, so it makes sense people wouldn't assume there has never been a blonde Baratheon without having a book that stated such. I would say, given the cutthroat nature of the nobility, had they had an idea of such an easy means of disposing the Lannisters, most would have taken it
The highborn peoples knowing means everyone would eventually know. Keep in mind that I'm approaching this as it being a flaw in the story's logic here. If one could simply browse a book and see that not a single known Baratheon has ever been fair of hair, then I don't see how that's something that's just randomly discovered 300ish years into the House's history. Especially when they're the house that replaced the Targaryen dynasty. They would be under a very large microscope, especially with consideration to who Robert's heirs would be, etc.
Another example of this is Rhaenyra's sons by Laenor. The Velaryons look like Targaryens in the source material, so Jace/Luke/Joff having dark hair is not that noteworthy, even if you want to say that those three were actually Harwin Strong's children (you could even say Jace is Criston Cole's son, the timeline allows it), there is less evidence to support that than calling Cersei's children bastards. There are plenty of Targaryens and Velaryons with dark hair, Rhaenys in the source material has a Baratheon mother and has brown hair, Baelor Breakspear has brown hair and his mother is a Martell.
Doesn't that only apply to Rob and the North, who were the only ones involved because of Ned's death. Stannis and Renly were already preparing before Ned died, and Balon only cared that everyone was distracted by the fighting.
It's clearly the main motivation for Robb and the Riverlanders to call to arms, but if you're asking me whether it's relevant to Stannis or Renly whether a bastard usurper had the hand of the king's head chopped off, I'd say yes? Especially to Stannis because he is vindicated by Ned saying he is Robert's true heir.
Sure, they would survive in the end, but they would lose any claim to the throne if the truth came out, which seems to be their end goal. It's not even like the Lannisters were the only choice... They could have joined the North forces, which probably could have let them take the throne, since Rob had no desire for it.
IDK, I think GRRM is trying to paint the Tyrells as shrewd and risk-adverse, though I mentioned earlier that them picking Renly at all sort of bucks that notion. It might be a longshot, but I wouldn't be surprised if the Tyrell's default position is to be distrustful of Northerners, mainly because the Starks received and raised up the Manderleys after they were exiled from The Reach following Aegon's conquest. So I still think the position that makes the most sense for them is allied with the Lannisters, which is where they ended up anyway.
IMO I don't see them being loyal to them much longer though, I don't see Mace living much longer and Randyll Tarly will likely pledge fealty to Aegon.