I sold or, even worse, dismantled my Kaer Morhen armor. I know it's needed to upgrade to all stages of the Witcher Gear armor. Is there currently any way in the game to get it back (i.e. buy it from a merchant or find the schematics)? If not, does anyone know if there's plans to patch it into the game?

There is only the one upgrade schematic for the Kaer Morhan armor (you start with the diagram), it is not needed for any witcher armor upgrades after that one.
Schematics for all the armor and swords associated with 3 witcher schools (cat, griffin, ursine) can be found by following various treasure hunt quests.

You can buy it back from the merchant you sold it to. Or if you have PC you could download the mod that unlocks the developers console, which would allow you to use a command that gives you the armor in question. I should also add, that as far as I know you can only upgrade this armor once, only the jacket, there is yet to be any more upgrades for it (but the game is still new, and huge, so not everything has been found yet, therefore there might be more upgrades for it but they have yet to be found).

It doesn't matter, you can get a better armor even before leaving White Orchard anyway.