What's the price difference between a new 36 inch and the repairs you would need to make for a 30 inch?

Presuming that the new range will remain centered under the hood, you have to look at your own kitchen and see whether the granite on both sides of the range could be moved in by 3 inches, without having to redo the countertops or the floor. Google "range filler trim kit"

You're idea is doable with cabinet fillers and countertop extensions. You'll probably want to do some sort of contrast, like a solid black, maybe Corian Nocturne. This isn't everyone's cup of tea, but you asked if it were possible.

You could probably have the better range for what I'd charge to make these changes.

You could put 3" fillers on each side or a inch one on just one side. You could even do them as narrow pull-outs. Or the 6" could be for storing cookie sheets or other flat pans. I wouldn't try to match the granite. I would make the countertop fillers from something else. Maybe stainless if you have stainless in other parts of the kitchen. They would be like an extension of the stove top.

The most cost-effective change would be to replace it with another 36 inch range.

Eh, once you've put in a 36" range, you're committed to a 36" range. Fillers aren't going to come out looking good. Moving the cabinets would cost too much.

Go 36".

Stick with a 36" range.

I probably would stick with a 36" too, unless you really feel that you can't afford a quality model and creating some filler would still be cost effective.

I am intrigued by the idea of stainless steel counter fillers on top of a pull-out. I wonder how much that would cost to have fabricated? One side could fit under the stove top edge, but you would have the additional seam between SS and granite.