The NATO countries had the experience of a failed ceasefire between the parties before, which was observed by OSCE monitors. The Serb's behaviour at the negotiating table also seems to have been unreliable, first coming up with a wildly unrealistic counterproposal set to anger NATO and then ratifying only a part of the accord when NATO said that it was indivisible.

All of this happened within the space of 6 days before the campaign started, as the wiki article seems to suggest. So the narrative that the Serbs compromised almost completely but their enemy was bent on war (mimicking the WWI narrative) doesn't hold up. Once you have an agreement on a civilian peacekeeping force, you have to negotiate about its size, makeup, rules of engagement, etcetera. So just proposing that you may wish to allow a civilian peacekeeping force isn't much of a step forward to the demand that 30,000 NATO troops are allowed in.

Whether or not there was an operation horseshoe is still uncertain. The actions of the Serbs during the war suggest that something similar existed. Of course, the existence of a plan doesn't necessarily mean that it will be carried out.