igen, az egy szék van is obviously too much, but is it really wrong?
Is 'az a szék' less correct, then?
Yes, because in the original sentence there is 'a' which is an indefinite article, which is equal with the Hungarian "egy", that can be omitted. So "Az egy szék" would be another correct answer.
"Az a szék" means "this chair."
"that chair", not "this chair", right?
Oh, yes, sorry, you're right.