"blueman"
Well, I plugged my dimensions into the "Shelf Sag Calculator"
(www.woodbin.com/calcs/sagulator.htm) using the following parameters:
Species: Plywood, fir (only plywood choice)
Weight Load: 200lb, uniform load
Note: this seems generous given that per the note, they
claim that a fully loaded bookshelf weighs 20-25lb/foot.
Which for me would have given a maximum of 96lbs.
Width: 46"
Depth: 12"
Thickness: 1.5"

The resulting maximum deflection was 0.06" which is about
1/16". Again, according to the notes, the eye can recognize a
deflection of 1/32" per running foot, which would translate into 0.12"
in my case of a 46" span, again well within my tolerances.

For comparison, a "standard" 36" wide Eastern White Pine bookcase with
8" depth shelves under a load of just 25lb/foot would give a
deflection of 0.13" or more than TWICE as much as my design.

ALSO, I am going to be giving each shelf a front edge facing of
1.5x0.75" poplar which should add strength, I imagine.

Assuming that all my calculations above are correct, I am now most
worried about shelf-pin tearout which is why I was thinking that the
wider pin (1/4" vs. 5mm) and the sleeves might help

I have found that 90% of the time, one a shelf is installed, people rarely
move it. And if they do, its only a little bit to accommodate a taller
object.
With that, I would only have a few adjustment holes in the general area
where the shelf is likely to be.

For very heavy load and thick shelves, I would use a 3/8" pin, set deep in
the side. To hid the heavy pin, I use a forstner bit and drill a 3/8 hole
in the side of the shelf and chisel out a notch in one side for installation
and removal. This hides the pin completely when in place.

As for the larger holes in the case, I cut 3/8 plugs and insert them in the
holes aligning the grain. If the ever need to be removed, I drill a small
hole in the end, insert a screw and pull them out for use in the now exposed
hole.

Dave