Interesting post, Turnbull.

However, I feel it stretches credulity to claim that Michael, coglioni notwithstanding, went to the party without an escape plan. What good is finding out the traitor if he dies the same night?

A possible explanation:

You've mentioned on many times the theme of Michael and Roth underestimating each other. Perhaps Michael was counting on being underestimated. If Roth et al. assumed that Michael was unaware of their plan they would not see the need to keep too close an eye on Michael throughout the evening. In that case, it wouldn't be hard for Michael to slip away at some point.

Still, I think that Michael would have been far more cautious than to place his life in the hands of such an assumption. It would be far more in character for Michael to have a detailed escape plan.

While there's no indication of what that plan might be, misdirection of some sort makes the most sense. For instance, an explosion elsewhere in the building or an assassination attempt on Batista would certainly distract the soldiers and cause enough commotion for him to escape.