The real problem with the force at Manzikert was that its size was in inverse proportion to its quality. Those Georgians, Armenians, Franks, Khazars, Cumans and Iraqis and others as well as a too small contingent of regular Thematic forces might individually have been perfectly good but mixing them together was asking for defeat. A small Thematic force, Varagians and cavalry might have been much smaller or at least a lesser variety and smaller numbers of mercenary forces.
Before any desertions and treachery their initial attacks were repulsed at great cost. Had they taken advantage of Alp Arslan's force failing to reassemble and the initial attack on Soundaq had gone better a better spirit would have prevailed among Byzantine forces. Ducas' treachery first involved spreading a rumour that the emperor had died but Byzantine forces were already in a fearfully disordered state after an order to turn and head back to an unguarded camp. The Seljuks fell on this disorder. The second stage of Ducas' treachery was his deliberate failure to commit the reserves.
Given that much of the Norman, Cuman, Iraqi and other mercenaries had found a better use for their time than fighting for Romanus and the manifestly degraded incompetent of the Greek corps the battle was not going to be won. A stalemate or something phyrric or doubtful was the best to be expected. But if Ducas had committed his forces Romanus would still have forces to meet the Turkish intruders that took Roman land for themselves. Romanus should have paid the Turks off and waited for a better chance and better forces. Ducas' treachery seems to loom larger because the Seljuks before the battle seemed almost to be routing but the main reason was that the emperor put unfit forces on to the field of battle.
It will be interesting to see how Crusader Kings simulates this. Mercenaries in Europa Universalis differ only in their names and a greater preponderance of infantry. Mercenaries in Crusader Kings will have to have unigue skill as they had then but provide a very substantial opportunity for treachery. A fully realised simulation of mercenaries must be part of the game or all its other features will be for nought - at least for the Byzantines.