The Firebird
Adventurer
Ok, I think I understand your concern here. Interestingly, it seems to me to be more GM facing than player facing. I.e., the issue isn't that the players are acting without a clear idea of potential consequences--they were. But its that the GM might have to make too many judgement calls in adjudicating those consequences.Applying the heuristic, have the world respond logically did not tell me what to do. It didn't tell me how the enemy faction might need to ration its resources (there are a wide range of possibilities here), what full range of magic items it would have access to (there are a wide range of possibilities here too), exactly what day they would mount their attack on (ditto), etc.
That said, I think "have the faction attack in accordance with its means and desire for vengeance" gives you plenty of information to work with as the GM without seeming arbitrary. Resource rationing can be determined based on the other conflicts the faction is involved in and how big of a threat the PCs are. The specific magic items and day of attack don't seem to me to be so important as to constitute railroading...I don't think it is railroading if the GM decides they have a staff of frost instead of a wand of missiles. Both the specific items and the number can be determined randomly if it would help.
In an ideal case, you may have a list of all the magic items available to each faction. This works better in a low magic setting.
The day of the attack can be determined randomly, or you can give a % chance each day, with a modifier for how conspicuous the players have been.
But, if it doesn't work for you, it doesn't work for you. This goes back to a comment I made earlier, that for you the phrase "the GM adjudicates the world" seems to be railroading.