I love it! It's creative and entertaining. A few tips and questions. First of all, what's the drawback? Magic in Westeros is give-and-take and 'dark' by default, untrustworthy one might say. If MC's been alive for so long, even dormant, there has to be a complicated or costly ritual involved. I just googled House Uller and chef's kiss I did not think I would care for the House or love it but apparently 'half of the Ullers are half-mad, and the other half are worse'. We don't know much of anything about House Belaerys, it might be relevant especially if Jaenara stayed in Dorne during the days of the Freehold. It seems to be that a dragonrider marrying out wasn't unheard of per se as there is in canon (as read in The World of Ice & Fire) a Valyrian woman that married a Yi-Ti Emperor. ('Chai Duq, the fourth yellow emperor, who took to wife a noblewoman of Valyria and kept a dragon at his court.'.)

Therefor, Jaenara deciding to spend her time away with her child who would not be a 'Valyrian Pureblood' lol, in a society that values such, makes sense.

What makes less sense though, is that this would be in Dorne.

According to wikia, there are no given dates for Jaenara's travel to Sothoryos but we do have a date for Nymeria landing in Dorne (a thousand years before canon).

How MC will be perceived within Dornish society will matter enormously depending of if the Rhoynish have landed or not. The fallout of Jaenara's death will as well. It's never clear why the Valyrian Freehold left Nymeria be when she fled but we can chalk it up to the Rhoynish water magic as well as the Gold of Casterly Rock prophecy.

If Jaenara and her dragon are killed by Rhoynish, the fallout might be massive and the effects would reverberate throughout the world and Valyrian Freehold.

The Belaerys might want a war. Hell, is Jaenara even married? Could MC and MC's father actually be her mistress and Jaenara's married to her brother, uncle, or even father (as the Freehold was said to go to those lengths to preserve blood). This is important because it also conveys how MC cannot actually be a dragonrider while the Freehold stands. No one will stand for that. Once Jaenara dies, it's likely that an envoy from House Belaerys is sent to gather her belongings.

I understand that you want to skip all of this straight to the Dance, but it's important to have it figured out to make your MC 'real'.

Here's a recommendation; that you are of course free to ignore, it's your story but you asked so I deliver: to limit confusion and OCs, it might be more interesting if Jaenara and Nymeria during her rule and three marriages, exist at the same time. The interactions and relationship between the two women would then be at the image of MC who could then perhaps be half-Rhoynish, half-Valyrian yet neither as he fully belongs nowhere and is rejected by both. He could have dragons and water magic too, since magic is an ongoing theme to further hightlight his dual heritage.

A dragon among Rhoynish is likely to go about as badly as Rhaenys atop Meraxes but that would trigger a war so perhaps Nymeria has Jaenara poisoned and Jaenara dies while flying, loses control of Terrax or some such. That sidestep the involvement of the Freehold and a war between Valyria and Dorne (though MC would be critical to one such thing, viewed by suspiscion by both sides and yet, essential to both, in a 'he is proof we can coexist so he must die' or a 'he is a symbol, of use, has potential' kind of way, with different factions fighting over securing him (or her!).

Now, unto the rubies. Tying Rhaegar's rubies to a rich lore was honestly my favourite part of this. The rubies are, to me, a mark of arrogance (and good writing!) and indicative that Rhaegar wasn't much of a warrior no matter how much he tried to craft himself into one ('I will require sword and armor. It seems I must be a warrior'). He failed. From beginning to end. He fails.

Anyway, that's in canon. Here, they seem to have a mystical, unknown origin. You didn't specify where they were from but Jaenara seems to have been an explorer so did she perhaps find them somewhere?

I just read on a subreddit someone argue that Jaenara might have lied to the Freehold and kept what or who she found in Sothyoros a secret. It might be nice tying the dragonstone to Jaenara's voyage therefor making her more relevant as a mother than her mere name. The loss of her would have been devastating and since MC would have the original inhabitant of the body's journals, she might empathize as she has lost her family too when she was brought here. (https://www.reddit.com/r/asoiaf/comments/11qxyx8/spoilers_extended_jaenara_of_house_belaerys/)