Tying a companion's/side character's quest into the main heart of the story to drive it narrative wise is not a new concept, and I find it bizarre that after playing RPGs for 20 years that this is suddenly a problem (Given your experience with Kotor, Bastila is one such example, Ciri from the Witcher 3 is another). I do not enjoy Act 3 one lick, but I do not go around telling people not to play or buy the game.
From what I can decipher of your post, it seems like Shadowheart did not in-fact have high enough influence (I might have completely misinterpreted because I found it a touch difficult to make sense of). She does not kill the Nightsong if you persuade her or let her make her own choice when you have done enough of her content and have high influence. Did she tell you about her past? Did she show you one of her memories? Did she say that your character is the only one she's ever felt that she can confide in about these matters? This was either a bug you encountered or as I mentioned, you did not know her as well as you think you did.
The nature of your post is not constructive, which is why people are reacting harshly. It reads like a rant and rambles in a manner that is a struggle to read. It sounds, at least to me, like you rushed through and did not take the time to do certain content that resulted in bad decisions and events that you otherwise wanted to avoid.
There is also a spoiler function in the full text editor here, btw, which might help with the construction of what you are attempting to convey.
(edited for readability)
