hey peachy,
you know more than me since you actually have the problem, luckily I've never encuntered it so not thought about it much.
You are right it is a bone problem but the bones are still growing but maybe it cannot be fixed.
I saw a case of crossbeak in an article but looking at it there...
inspiratinal story but that little one like you say had some underlying issues and still has. Will it be ok with such severe crossbeak? I've never seen it so bad.
Crossbeak is caused by a deficiency from what I've read.
If you have not yet given it some bakers or brewers yeast I would recommend...