"It is a wall."
Translation:Είναι ένας τοίχος.
I don't think so. Although είναι is a verb, I'm reasonably sure that you don't ever drop into accusative after using it (or any of its conjugations). A native speaker might want to explain a bit further though as I don't really know the actual reason why.
That's right. είμαι is not a transitive verb -- it's a copula, which joins a subject with a predicate that says something about that subject and refers to it.
Since it's basically a form of "A = B", it makes a certain amount of sense that "A" and "B" are in the same case -- and that's indeed what happens in all of the languages I know (though I wouldn't be surprised if there are languages that violate this).
Rule of thumb: use nominative on both sides of είμαι.
Another verb which acts similarly is γίνομαι: Θέλω να γίνω γιατρός (not: γιατρό).