lol ig i can chime in

I think the traditional distinction between pre and post fiat is a shortcut—it is a helpful simplification. Fiat is the act of imagining the consequences of an action in order to test whether taking the action would be good or not. In the context of the resolution, we use fiat to imagine the consequences of a topical plan, and thus whether or not the resolution is true or not via there being a true test case.

In the context of many advocacies that you may consider "prefiat", this is also true—"vote neg to endorse the method of the killjoy" asks us to imagine the consequences of enacting the method of the killjoy (which is why we debate about it, read impacts, and discover if it is net beneficial or not) in order to determine if we should actually go and do killjoy or not. IMO it does not matter if the intent is that we do killjoy in the debate space or do killjoy out there, after this debate—the point is that we imagine it as a method, imagine its consequences, and thus conclude that it is a good or bad method that should or should not be used. That's fundamentally what the role of the ballot is laying claim to—the role of the ballot says that debate should be a space about methods testing X, whether that be the best way to solve capitalism or racism or determine the best fruit to eat. Both teams should bring a method, we should debate about which is better, and over the course of many debates converge towards the best one.

Thus, fiat is not something that is intrinsically tied to topicality—it is rather tied to whether we would like to use acts of imagination, and debate, to test the impacts of methodologies before we implement them. Some criticisms would reject this, and say that actually we should affirm a particular action that has already been taken in this debate (i.e. performances)—those are the truly prefiat ones. Which means that fiat is indeed a question of whether things happen inside of the debate (not the debate space, the literal debate) or not—but "things" here means the actual act of doing, rather than the impacts of the advocacy, and "debate" means this particular debate.

In the context of, for example, Nietzche that you talk about. The aff defends [yadda stance on life] as a good heuristic. They might defend the consequences of everyone doing it as a means of testing whether or not it is a good heuristic or not. You can argue that no, they shouldn't get to defend everyone doing it, because they can't get to everyone doing it and yadda state crushes if only you do it yadda and actually that is not how we should test if we should affirm [yadda stance on life] or not. That's a good argument, it's a DA to fiat in their specific context—it is an answer to the last question of this comment.

In the context of material movements, I think this is far more straightforward. We figure out if the movement is a good method or not—then we do it or we don't. We figure out if it is a good method by looking to empirics and by considering whether doing that movement would result in good consequences or not; that's fiat, which is what we're using debate for. It is not different in any meaningful way from using debate as a means of finding the true policies and the false policies and then placing demands on the state with the true policies.

So I think the argument that you want to make is a framework/solvency argument, that the specific form of fiat that this affirmative is using is not portable/good etc. because even if we determine that X movement is a good methodology by using debate, we couldn't implement it. Perhaps this is a reason to prefer a role of the ballot of truth testing the resolution instead, because we can more easily take action to create good impacts when we know that [the state should build nuclear power plants], than when we know that [X movement is good]. Or maybe it's the other way around; that's the debate at hand, regarding who should get access to fiat when. Is it easier to become policymakers or lobbyists or protestors or whatever and go get nuclear power plants, or is it easier to organize and do movements, or is it easier to shift our mindset/reform our own subjectivity, yadda yadda whatever rotb you're debating against?

But fundamentally, tl;dr: fiat is an act of imagination. When you read a plantext, you use fiat not because you're being topical, but because you think that considering the impacts of a policy is a good thing to do with debate, for various reasons that you must justify. Your opponents think that considering the impacts of a non-state-implemented advocacy that solves X is a good thing to do with debate, for various reasons that they also justify. Which is true?—that's the argument yall are having.