Leaving out the accent, when it's required, for the reason that it's too much work is a bad habit that's going to be very hard to break later. I think Duolingo is right not to accept it, and I think it shouldn't let it slide even in the earlier part of the lessons so that students, especially new ones, don't get so used to doing it. After all, "esta" and "está" are two different words; it's as if saying that, in English, using "this to mean "is" should be just fine. Just my opinion.
Commandant and commander are two different positions, at least in the US military.
A commander is a US Navy rank (O-5) or the officer assigned to be in command of a military base.
A commandant, however, is a position that tends to be the head or second-in-command of a military training facility. In the case of West Point, the USAF academy, etc... they are the second-in-command. But for each service's War College, they are in charge.
Is "commandante" a rank in Spanish... or a position?