YOU'RE ONTO SMT
Yeah I think it was Mitsuhara at the beginning too. I reread that little part and it talks about women being property and him falling in love with someone he shouldn't have which fits the bill for Mitsuhara.
The woman being property and things like that sounds very noble like, using arranged marriages to gain power and stuff like that, so I feel that part fits Mitsuhara, whos a noble and has been raised to think like that (even though he might not actually think that way, but that's the way of thinking of his parents n stuff)
And falling in love with someone he shouldn't have, aka a woman he already knew would never love him because she loved another man. Plus a dying woman at that.
Then it finished off with he was very happy, which again, fits Mitsuhara, even though their time together was short, she changed his view points a lot and he genuinely fell in love with her and was very happy for that short amount of time he was with her.
It also tracks with the whole letter thing. Shinpei knocked the letters out of her hands, so they were most likely still there after everything happens and maybe he held onto them cuz they were the last remnants of her, because honestly I don't think she's gunna survive this. I think its gunna be a tragedy where they both die and Mitsuhara hangs on to the letters for the rest of his life to remember her, even though the feelings inside the letter weren't for him, which is another tragedy within itself.
But yeah, I gotta agree that it's him at the beginning and not Shinpei.
What if the person at the beginning of the story was Mitsuhara? What if Satoko survives this and chooses to run away with Shinpei this time? Because Mitsuhara is experiencing his first love with Satoko but she’s in love with Shinpei and sees now that she can’t leave Shinpei behind!