I know the syllable structure is cor-nel-sen
So it should be pronounced with a short "e", isn't it?
Or maybe because it is a last name the rules don't apply here?
Cornelsen is a patronymic name as for example Petersen = Peters Sohn = son of Peter.
Cornelsen is derived from the first name Cornelius, which is of Latin origin and has similar accentuation of the e.