Let's not just auto-reject all evidence to the contrary and assume the Velaryons are the classic Scheming Mad Vizier Clan who will automatically fuel a civil war at any opportunity and for any pretext. At least not until we've actually put some effort into sounding out Rhaenys, because I'm pretty damn sure Rhaenys can act as a check on Corlys if she wants.
Which is why we need to talk to her, get her onside in whatever capacity that ends up being, and then talk to him.
Corlys isn't an idiot by any stretch of the imagination but he's a extremely prideful man - he's a noble to begin with but then this is the guy who took a minor house to one of if not the flat out richest along with literally being one of two dragon riding families basically single-handedly.
If the play isn't there the play isn't there as long as the chance was given and he was acknowledged but nothing will get this man more riled up and willing to back someone with a chance against Rhae and Viserys then being fucking IGNORED.
From his perspective Rhae and Viserys stuck up their nose at Viserys marrying Leana and then have literally completely blanked him in the YEARS since. Yeah he was the one that stormed off and went sulky, WE know that, but to HIM he reacted reasonably to being turned down rudely and then has been flat out ignored since.
He's ambitious and arrogant but intelligent and clever - he can take a "fair" loss and setbacks at the table but when from his perspective he's not even offered a place?
Daemon can marry his daughter, is a strong warrior, is the second most powerful dragon rider (behind Corlys wife), is charismatic and cunning and is the brother of the King against the daughter AFTER they've just spent several years fighting together in the Stepstones.
By any consideration that has potential and when the other side have seemingly decided to ignore his existence suddenly it looks like a viable option.
Especially when the Baratheon head has a grudge against Rhae - between that and Rhaenys connection there's one Kingdom.
IIRC the Lannisters aren't a fan of Rhaeyra - promise them Deamon's heir to one of their daughters and there's another Kingdom (minus Joanna's family)
The Reach is the Reach so in any civil war any side can guarantee at least part of it (and even more of it if Dorne looks like it's going ahead)
Same with the Riverlands and definitely at least one of Bracken/Blackwood (probably Bracken)
Plus the obvious massive dragon imbalance
And that's just immediately obvious and low hanging fruit.
Yeah normally it wouldn't be enough to begin with for someone like Corlys to risk it all and take it on and see what/who else he can get onside but when he's got his back up and is pissy? When it's literally this or "nothing" (i.e. what he currently has) and to him he wasn't even given a chance?
Even WORSE if Dorne and then Strong and Otto are voted after.
Like talking to Dorne first - okay it's one of the "Kingdoms", he wouldn't like he's not first but he'd understand and it wouldn't
really bother him.
But if Viserys and Rhae talks to House Strong before him? To OTTO before him? Being considered behind House Martell is one thing, but being behind a Riverlands vassal house and OTTO FUCKING HIGHTOWER? At which point he STILL doesn't know if he's (i.e. Leanor) is going to be considered and talked to at all?
Honestly at that point even if they were finally voted to talk to I wouldn't be surprised if he ignored it and didn't even consider it just from being thought of AFTER Strong and Otto.
Rhaenys and Corlys being talked to after Dorne is fine - I voted Rhaenys just because I'd PREFER it first - but if you guys vote Strong and/or Otto before them I'm going to start thinking you guys actively want a Dance against Deamon and Corlys.
However, that all being said - the Gwayne option would be the easiest one. And it would ensure our children are Targaryens, no questions asked and no silly compromises.
Honestly that has to be bottom line hard requirement for ANY marriage option, not even something done for concessions, just "this is happening". It's absolutely wild to me GRRM wrote in canon that her strong children with Leanor were Velaryons - given how central Targs are and as literally the LAST Dragonlords of Old Valyria the fact Viserys agreed a patrilianial marriage is absolutely batshit insane.
Really makes me doubt that Viserys ever actually thought she'd end up Queen with allowing that.
And fuck, if they get uppity not only is just "well no then" an option but bring up the fact it wouldn't even be mentioned the other way around. Like if this was marrying their daughter to the Crown Prince they'd never consider the children having her name.
i.e. "You're marrying the heir to the Last Dragonlords of Valyria, why would you even think we'd let the crown pass out of the name? The heir being a daughter is irrelevant. The Blood of Old Valyria supersedes all"
Even ignoring my desire for equal inheritence - this should just be bog standard for all the major houses. Of COURSE the children of a female heir/ruler of one of these houses would have her name.
The Starks are 8000 years old - you REALLY telling me they've NEVER had a ruling Queen?
I'm pretty sure even with the Durrandons and Orys when Aegon originally offered Orys he would have ended up taking the Durrandon name and Argella would have eventually been the Lady Paramount with Orys as the consort.
It was only BECUASE they ignored it out right that after conquering them Aegon went "well fuck you then, your daughter is still going to marry Orys she's just going to be a Baratheon now and he's going to have the seat".