We generally think that headcanons are fine, provided they don't clash with anything demonstrated in canon. From how you've described Casey's situation and your headcanons on his background, it sounds like you've put sufficient thought into this and in making it mesh with canon. Feel free to add it in if you want!

(thethrillof suggests making a separate post in your journal specifically for the headcanon parts of his backstory, but it's up to you how to handle it.)