Battle Fury is not a separate action though, and incapacitate does prevent Battle Fury attacks according to the Rules Supplement, demonstrating that incapacitate will affect an action already in progress.
You are correct that steps 3, 4, and 5 are not themselves actions. Neither is step 1. They are all results of an action and a part of carrying out that action. It is clear that incapacitate will prevent step 1 in an additional attack like Battle Fury, and thus the entirety of the additional attack for without a legal target the attack is cancelled out right. However, my hypothesis that it affects the other steps is conjecture. Saying that they can not be affected though because they are not actions does not have merit because the same would apply to Battle Fury, Sweeping, etc where we know they are affected.
Whether Turn to Stone stops an action in progress also seems clear that it does not, as incapacitate does not prevent steps 7 and 8 from occurring if you become stunned during step 6. So, it is clear that you continue the action and it is also clear that some steps are affected while others are not. We know for certain that steps 6, 7, and 8 are not affected and that step 1 is, but we are in the dark about all of the other steps.
The reason that I hypothesize that steps 3, 4, and 5 are affected is that it seems to me that more than step 1 needs to be affected for there to be a reason to restrict Turn to Stone from being revealed after step 1. If it's only effect would be to prevent additional attacks from occurring (and mess with your defenses and armor in the case of a counterstrike), I do not think there would be much reason to prevent it's reveal during an attack. It seems to me that the designers intended it to have more of an effect, even though there's no written rule stating such explicitly. The only rule implying it is an ambiguous interpretation of what it means to take an action which has been shown to include having an effect on actions in progress. As steps 3, 4, and 5 represent the primary functions of the attack, representing what you are actually doing, it seemed logical to me that they would be the ones affected by incapacitation. The steps unaffected would then be the ones primarily carried out by the defender rather than the attacker.
So, it all boils down to our interpretation of taking an action. A strict interpretation of simply meaning the initiation of an action has been shown to be false, as it has been clearly stated that it prevents certain steps from occurring during an action already in progress. One could take a strict rules as written approach and say that it only affects steps clearly mentioned, and thus only step 1, but I believe there is evidence that indicate the rules as intended is for it to affect more than that and such an interpretation does not contradict anything within the rules. In the end though, we need an official response. Until then, anything else is merely a house rule.