Okay, so here is a couple of break down.
1st, let stop saying Aesir's leading her on, because that's not what happen at all, like we are both agree that's Kaeha's action is insane yes? (we will talk about how the author writing her later) Aesir may have been a 30 years old man when he reincarnate, but he lived in the high elf forest for 120 years and was taking it easy, no sane person would interpret his parting word like she did. And he recognize this later in ch 20 regarding his deeds and words, but by that point shit already happen. He wasn't leading her on as much as comforting her when he is parting and just say what he think is right in the moment, we know this, cause the author put his thought bubble up for us to read.
2nd, okay now let's talk about how the author wrote Kaeha. I honestly think you give the character less credit than she deserve. Aesir's words may have put a curse on her, true. And use your word, did she tunnel vision to dedicated her life for him? sure, it's love, love is blind and consider the thing he did for her, she was madly in love. But on the other hand, she also put a curse of love on Aesir, he will never forget her, for however long he may live and it is an eternity. Like, was the author wrote her just to be a love interest? Yes, but every character has a role in the story, hers was the first love for MC, and she gave him one that he will never be able to forget and he may never even love or getting too close with other person again. And again, we are viewing this only through Aesir's pov, i don't know why you keep insist on she being "a husk of a person", was her love for him heavy? hell yeah, but she is also a sword master of a dojo, mastering the sword and expand the dojo is her job, the iffy part is only her motivation for doing so. On the flip side, if you can look at on a more sinister angle, ask yourself this, how do you make sure an immortal will love and remember you forever? Kaeha's answer is basically, doing something so insane and giving everything for them, was it logical? No, she did this in her 20-30s. But she did succeed, judging by this chapter.
Now, could the author fuck it up and introduce 2nd,3rd love interests? could the author just make it so he basically forgot all the lesson he learn the past 20 chapters? He could, but we are talking about what currently happen in the story and the people reaction to it.