Am I being dense? I think I'm right, but I can't figure out why.

me:

let's go take a nappy.

sr. popcorn:

that would be the best

although idk how much i could actually sleep right now

me:

ok well you could rub my hair while I sleep.

that was not grammatically correct but I'm too tired to care.

sr. popcorn:

i dont see an error?

me:

I think it should either be: "you can rub my hair while I sleep"

or "you could rub my hair while I slept"

right?

sr. popcorn:

slept is past tense

could works

sleep is right

first option is best

A is the best choice

Doesn't could as a future conditional have to be followed by the past participle or something?