Of headcanon
I think it's generally agreed that fighting about headcanon is pointless. Live and let live and all that. However, you can't just make shit up and say that nobody can argue about it because headcanon is sacred. In order for an idea to qualify as headcanon, it needs to work with actual canon. That's the whole point. It's something that's not explored in canon but that would work and be cool.
For example, saying that Prussia and Vietnam are friends because lots of Vietnamese immigrants came to East Germany is an interesting headcanon. We've never seen them interact in the strips, but there's nothing that says they couldn't have some kind of friendship.
Saying that Old Fritz raised chibi!Prussia into an adult doesn't work. It's a cute idea, but no matter how much you insist that it's your headcanon, it won't magically become plausible.
For example, saying that Prussia and Vietnam are friends because lots of Vietnamese immigrants came to East Germany is an interesting headcanon. We've never seen them interact in the strips, but there's nothing that says they couldn't have some kind of friendship.
Saying that Old Fritz raised chibi!Prussia into an adult doesn't work. It's a cute idea, but no matter how much you insist that it's your headcanon, it won't magically become plausible.