Ancap philosophy generally points towards a poly-centric legal system which allows for differing notions of theft/ethics/values etc. to exist within a polity.

This is a little misleading though, because no matter what differences there might be between court systems, there would always have to be a foundation built upon ideas of private property rights, commerce etc.

Depending on how someone choses to define theft or rights, you could make numerous arguments about how ancaps are stealing from the peoples of the Earth by claiming exclusive monopolistic rights to scarce land resources.

Consistency can't be a bad thing in itself, since the whole art of political philosophy (whatever strain) works very hard to make things consistent. Political philosophers across the political spectrum make arguments that, at the end of the day, are often founded on appeals to consistency.

Well, to be fair there have been many thinkers who have challenged ideas of truth and consistency, such as Nietzsche, Foucault, Deleuze, de Sade, Artaud etc.

I think that the objectivity that ancaps attach to their claims is pretty dangerous, because they actually believe that they have all the answers. And a priori no less. How isn't this a similar mindset to ideologues throughout history, including the church, authoritarian communist regimes etc. who also had access to a certain objective knowledge that the rest of us were just too ignorant to see?

There are plenty of ancaps that have reasonably conventional views on morality but who reject claims about political authority (and reject collective authority--i.e. syndicalist style democracies).

Rejecting collective forms of governance is fine, in fact depending on how you define collective I might agree fully.