What was the rationale for the user-input `maxk` providing a minimum to that value?  

In the case of `maxk`, I think the most usual case will be if we know that everything we care about in an image is bandlimited - i.e. we know it has a maximum `maxk` - but that in fact the contents of the interpolated image are so smoothed that you can get away with a lower effective bandlimit.  Ground based images would be a good example, or space based images with pixel noise or defects (which are not bandlimited but ought be ignored for describing the galaxy).

In short, in the cases when you know a `maxk` in an image and want to tell GalSim about it, you will want to set that as the maximum I think...