I see. It would be possible to avoid it if he refrains from returning to the capital, but... unless he is coronated, I suspect the kingdom would immediately fold to the empire under its current ruler.

[Folding his arms, Byleth hums thoughtfully.]

Assassinating Cornelia before his return is also a possibility, but we can't know for certain if she's the mastermind or just a pawn. Given the timing, I suspect the latter.

[In which case killing her off would accomplish nothing other than replacing the figurehead, and said figurehead might opt to kill Dimitri off sooner. He shakes his head.]

I don't think there's a way to guarantee his survival if those circumstances are changed. Escaping an execution is quite the feat.