Shouldn't it be haluaisitko?
Indeed it should! Remember to report if this one comes across again.
Isn't it like "haluatko" means "do you want" and "haluaisitko" more like "would you like to have"? Like more polite version or something...
Yes, that's correct.
Saippuaa with 2 A is because is uncountable?
That's right. It's in partitive because you're not taking the whole soap but just a little bit/undefined amount of it.