@Tanaka did nothing wrong so you don’t think there is a best case scenario where Romanos IV successfully minimizes the Doukai ?
Well, in my view at least, as long as Romanos is trying to push for a change in the dynastic order, there's going to be a political crisis, which is most likely going to result in civil war. With the Doukai being the weaker side perhaps, the possibility of them inviting the Seljuks to distract/weaken Romanos becomes rather likely. So, there's a good chance that even if Romanos manages to enter Constantinople in the end, any progress made in 1068 - 1071 would be undone to a large extent; worse, surviving Doukai partisans and allies could continue fighting for some time, and other figures smelling blood in the water are going perhaps to try to usurp the throne, especially if Romanos is facing troubles.
A success would require the coup in Constantinople to fail as well I think. This may be rather difficult because the Varangian guard and other local units were opposed to Romanos because he didn't show them the same interest and didn't try to cultivate their favour, unlike his predecessors. For example, when Romanos' ascension was announced, iirc, the Varangians stated they didn't want him as emperor, and it took Michael's personal appearance before the guard to pacify them; and in 1071, the guard units were the ones who energetically supported the coup staged by the casear John; this means that the Doukai are most likely going to secure control of the capital in most cases.
Perhaps if Romanos had managed to win over the guard units, or if a trusted commander could happen to be near Constantinople, the coup could have been suppressed quickly enough; John Doukas miscalculates, Eudokia Makrembolitissa and others don't side with him, nearby units approach Constantinople. I guess that in this case, John Doukas would probably try to escape, which creates problems of its own - imagine him managing to arrive to Robert Guiscard's court. Therefore he has to be caught and neutralised, and the same most likely applies to his sons. Romanos returns to Constantinople, having managed to eliminate the Doukai opposition. But this doesn't mean his problems are over: the thing is, he is trying to impose a new order, and no matter how distant Michael is going to feel, he's still the emperor and he is most likely going to start feeling resentful; Eudokia Makrembolitissa, although no Zoe or Theodora, who could count on the veneration of the Macedonian dynasty to defeat the attempts of Kalaphates to push them away, is still an influential figure and were her position and that of her children continue to deteriorate, she could spring into action. With the Doukai gone, she actually has a greater freedom of movement (because her other major concern, John Doukas, is gone); other aristocratic families are probably going to try to take advantage of the political situation, especially if they view Michael as someone easy to be controlled. In the scenario closest to this situation in my view (the reign of Romanos Lekapenos), Constantine VII moved after the removal of Lekapenos to seize full control and he succeeded in doing so, therefore I don't think it's out of the question for Michael or others close to him attempting to do the same with Romanos' sons after he's no longer in power, which again opens the floodgate for civil war and the questioning of the legitimacy of the new emperors.
