I think "έχεις" should be accepted, since there is no indication in this phrase of emphasis on the you.
I agree. There should be two correct solutions. The English word "you" can be plural or singular. So 1 and 3 are both correct.
I had to mark all correct translations, and two of three were almost identical. The only difference was a space before the final fullstop: 'Εσύ έχεις.' and 'Εσύ έχεις .'
Which "very obvious error" are you referring to? A few things have been brought up in this thread so a little more information is necessary to understand which one you're pointing to. :)