Preposterous and coherent are two different things. Coherent only requires that the Amish understand the rules to their conservatism. The bible being sometimes incoherent (and always preposterous) would not stop the Amish from being coherent so long as they are aware of their interpretations.
The Amish core beliefs are roughly:
nonresistance (pacifism), community, humility, and separation from the world (Gelassenheit), seen in plain dress and limited technology.
That's a coherent set of beliefs that they have been following for around two centuries.
Now of course this is all in theory as I am not Amish nor know much about their communities. For all I know the Amish could be racist and then I'd have to agree that's not coherent.
However even if there are problems in practice you can see that conservatism can be defined.