Responses
But i think they said they started treating her as soon as shinpei left the room, but when he left he was just (presumably) slowly walking home in a trance, where he then stayed for a bit so all they had to do was quickly run outside (either the bodyguard or the duke or even anyone there) to catch him

So first things first this is a personal opinion! You’re free to disagree with me just please be nice about it >_<!!!!
Anyways I think the ending is fitting, first off this is tagged with the tragedy genre, and the specific point of is not her dying, but them losing the time they would have spent together.
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So based on numerous of comments I’ve seen, you have to remember that Shimpi was more than likely nowhere near them for a long time, since he was out traveling around. Another factor is that they have no mail, so how would he get any letters if he’s a nomad essentially. Even if there was people looking for him the search would be too broad and would be in vain, since he did leave that area as the manga implied.
Don’t get me wrong I do think they looked for him, but looking at the timing and how quickly he left, the searches would turn up empty. You cannot find someone who doesn’t want to be found.
Plus for his character it makes sense he indulged himself trying to make up for his sins.
Why would you want to go back to a place where your pain is still so fresh. It adds in that he only went back since it would be his LAST chance to see the time they spent together. Again it fits into his character choosing to avoid the area all together to save himself from the wound it left.
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The tragedy aspect of this is that:
He believed her to be dead, self repented for years, became quite literally became a shell of his former self without her. Whilst she was stuck rooted to one spot holding hope that someday he’d return never knowing when or IF he would. And now only have the last of their lives to spend together.
That’s why it’s a tragedy guys ….
I think it was splendidly tied together with how the viewers are reacting, because that’s the point of it/the intention. However that’s just how I’m viewing it though ( ̄∇ ̄")