Try upsampling to about 300%, and then run the PS "diffuse" filter a few
times.  This will make the shape of the pixels and the noise a little
more complex.  I often do this to get rid of pixelation in low-PPI
prints.  The center of the deformed pixel blobs will still be pretty
much the original center, though.  What would be better yet is some kind
of filter that arbitrarily misplaced the center of the pixels, sort of
like arbitrarily extending some of the upsized pixels in various
directions, and shrinking the remainder; IOW, make the sizes of the
original pixels vary, and offset their centers, to get rid of the
gridding.  A large, very complex grid mesh for the liquid filter might
accomplish this.

This distorts the noise and the image.  If you just want to distort
noise, to be more grainlike, you may as well remove the noise and start
from scratch, or subtract the denoised image from the original, if you
want to work with the original noise.  I don't think there's anything
particularly desireable about sensor or digitization noise, myself.

Not true... if the neg has grain, when making large prints it is always
important to make sure the grain is sharp, expecially in the print's
corners.  Anything less looked very sloppy.

Not true, ISO 1600~3200 digital noise is easily reduced to very tolerable
levels with many programs, some of them free.  Even the worst digital
noise can be minimized to a level an old film shooter would feel is
better than bad grain.

Digital noise appears the reverse of film grain, because usually it is
apparent only in the dark areas; film grain is more objectionable in the
light areas of an image.  The consistency you want is a quality step
backward, if you're shooting digital.

Generations of film photographers happily accepted grain as the price of
speed, so there's no reason why digital noise should not be equally
acceptable for its own particular texture.  Each medium has its own pros
and cons... one does not fault an oil painting for being thick and
"lumpy" in comparison to a flat watercolor.

From the aesthetic point of view, I often prefer a bit of digital noise
to give a certain flatness and paper-like "tooth" to an image, unlike the
3-D slickness associated with ferrotyped prints