Картошка is uncountable, it always uses the singular form. It might help if you think of it as "some potato" rather than "a potato".

No reason, except it is a formal word people do not actually use in speech (it is something you would read on a price tag). We prefer to stick to картошка, which is what you can hear.

I'm pretty sure it's accepted. But if not, you can report it, because it's correct as well as "свари".