https://www.youtube.com/watch?v=OX5iGh2pgxk

this bit of a song I found has the freedom motif https://youtu.be/7gVj0d8HnBU?t=170

lol

It's because at that point in the game lancer and king are serving the knight, and it plays when the knight takes Undyne

king has never met the knight canonically

again "they serve the knight" you know religious style

not only lancer has it

this plays during the undyne kidnapping too

No, it is because anything that has motifs from Black Knife is related to the knight.
If lancer were the knight, why would he help us survive King in Chapter 1
And they SERVE the knight, so they are related just none of them are the knight.
The knight is 100% a Lightner, we know that from Queen saying "Is this a power all lightners possess?" And I believe that Darkners function the same as Undertale Monsters, where they don't have enough " Determination " to persist after death, and not enough to make a fountain.
And anyway, Darkners petrify after they are not needed in a world or even in their own (shown by the Zapper turning to stone at the end of Chapter 3) so the knight would petrify after taking Undyne as it wouldn't be needed until Chapter 4.