I think basically all of this makes a whole lot of sense, especially with the glaring sense of inadequacy Lyon has. I mean, no, he's not physically strong, but he's shown to be extremely intelligent and talented with magic. So having Vigarde foster that sense of "you're not what I wanted", followed by meeting people like Ephraim and Eirika who are absolutely his father's image of perfect royalty, could definitely set up those deep-seated insecurities.

On his own, I don't think Lyon would be unstable enough to really go off the deep end - I think the evil vibes off the Stone started amplifying his angst before his father got really ill (since his death is a total surprise to just about everyone, it probably was a sudden decline), so must of what we see in-game is like an amplified version of all the fears he had to begin with - but instead of resenting his father (a la Zephiel), he becomes desperate for his reassurance and approval, which probably factored into the whole resurrection thing.