How would this ko play out?

In the game

white could still attack at Q18 which I think would give black the choice between seki and thousand-year-ko. (Obviously, if this is a reading error, you can tell me so and ignore the rest of the questions.)

In the current game, white wins anyway, but suppose that black is just two points ahead so that seki is not an option for black and both players need to win the ko to win the game. Who would play the approach moves? And who would win the ko?

I think you’re misreading it… play out the sequence of W18, B Q19, W P19, etc… ultimately B has necessary liberties to capture W…

I started with W Q18, B Q17, W P18.

ok, you’re correct… this will inevitably become seki unless B chooses to resolve it earlier in the game