Thanks to jireir, for the suggestion to add spoiler tags to my post which I have now done. Sorry for not having thought of that myself and I hope I didn't mess up anyone's enjoyment with my thoughtlessness.
I have also taken their advice and gone ahead and forced myself past my discomfort and have gotten deep enough into K2 for the characters to hook me, though my concerns and misgivings remain. The writer / developer still has their talent for making compelling characters and making them easy to get attached to, even if that doesn't always lead to a happy or satisfying outcome as with K1.
I am encouraged by the replies, it's good to know I'm not the only one who feels this way. It really feels like the last portion of the game is incredibly rushed and none of the developments are, well, developed. Instead they're just dropped on us and we're expected to accept them even though they turn everything up to that point on its head. There's nothing wrong with a twist, but it can't feel like it comes out of nowhere nor should it lack adequate setup. For a twist to work, it has to feel like it was always that way and just wasn't apparent and once it drops it forces you to reexamine a lot of things that suddenly make sense in the new context, and see them in a new light. Sadly, that's not what happened and instead the twist feels flimsy and contrived. It loses its impact because it doesn't feel believable and the jarring nature of it rips you straight out of the story.
I cannot accept Hikari's sudden about-face on her love for Rumi, nor can I accept Rumi's abrupt change of characterization that essentially rendered her a completely different person for the sake of what feels like a shoehorned excuse to wrangle the plot in a certain direction whether it made sense or not.
Please don't misunderstand, I am not slagging on the author nor am I hating on their game or its story. There's so much good to love here, if there wasn't I wouldn't care enough to write so much about it I'd instead just move on. Trash isn't worth being upset over! Instead what I see here is a gem marred by a fatal flaw, an incredible journey spoiled by what feels to me like an unintended drift off course and a sudden need to course correct with no time to establish it properly. Again, if the characters and their story weren't compelling then it wouldn't matter, it just sucks that it was sooo damned good right until the point it imploded and squandered all that wonderful potential.
If I could give the author advice, I'd say go back and split the prequel into two parts and make the second one focused on the Miru romance and do it in a way that doesn't throw Hikari or Rimu under the bus but instead develops it organically with the same kind of loving care the Rimu romance got. Take chmo45645's suggestion and do things that way, and the objections I and others have would disappear. The story would then live up to its awesome potential, which I think we all want.
