Manzikert was hard yes, but terminality was caused by the IV Crusade diverting to attack and conquer Constantinople in 1204 (note: i'm Catholic but I still see this as a mistake, up there with the Crimean war)
The Fourth Crusade did, indeed, quicken the demise of the Roman Empire. There are some historians who argue that the Roman Empire actually ended in 1204 and not 1453.