"An entire month" should be marked correct if "A entire month" is allowed
"a entire month" should not be accepted.
It was our mistake! Corrected :)
On another discussion it was said that ολόκληρος comes before the article.
Is it right to say that it comes before a definite article, but (as here) after an indefinite article?
Yes, I think that's a good explanation!
Could "That's all!" be translated as "Είναι ολόκληρος!" for something that's finished?
That would be "αυτό είναι όλο", I believe.
That's right ;)