Calling a giant monitor you sit in front of 'retina', even if it is 4k, is just wrong IMO. All in the density. But yeah, I don't see the point in iPad going higher. Once things get into the 250ppi bracket I can't really tell the difference anymore at any viewing distance.
Not necessarily. There's a theory that retina isn't a specific resolution - Apple have never said what resolution = retina. It's just the resolution whereby you can't distinguish between pixels, but that resolution will differ depending on the size of the screen, or even how far you are from it. There's been suggestions that Apple will call their 4K monitors "retina", even though they're not the equivalent resolution of the ipad or Macbook retina.
A Retina screen on the iPad mini would be even sharper than the 'normal/proper' one!