TBF the error can become that big if you do a bunch of unstable operations (i.e. operations that continue to increase the relative error), though that’s probably not what is happening here.
NOTE: When Qt is installed on the system, the native style uses Qt’s QStyle to achieve native looking widgets.
I’m not that familiar with KDE’s styling, but if I remember well it should just be a Qt style, so it should work.
Regarding rewriting Dolphin, I think in theory you could do that, in practice it’s probably pretty challenging given the amount of features Dolphin has.