What was the rationale for the user-input maxk providing a minimum to that value?

Not much I guess.  The main thought was that for a RealGalaxy, we know the convolved image must be larger than the PSF, and hence have a smaller stepk.  I didn't think as much about maxk, but I guess you're right that in that case, we know the convolved image cannot have maxk larger than the PSF's maxk.  So a maximum would make more sense.