Bread – Why do microwave ovens make bread rubbery

I'd say your problem is that the yeast has exhausted the food in the dough before you even get the leftover dough in the fridge. When you pull it out the next morning, the yeast does nothing or very little because there are no more sugars left for it to eat. And not only will this prevent rising, it will also limit browning because it's at least partly the sugars that brown in the oven.

You could work some more flour (and water, to maintain hydration) into your dough as yeast food--maybe even a little sugar if you want--and then rise/form/rise and bake. But at that point you're doing what @tmow suggests--just using your pizza dough as a starter for new bread.

Marshmallows expand so much because the water in them becomes steam, and gas takes up a LOT more volume than liquid. Specifically, 1 mL of water becomes ~1.36 LITERS of vapor, before it gets heated further. That's 1000-fold expansion, before you add additional expansion as the gas is heated.

Marshmallows don't have all that much water content, but when it's trapped in a stretchy gelatin matrix that holds gas readily, it only takes a bit to blow the whole thing up like a balloon.

You are correct that the gas expansion on its own is insufficient; unless I've badly muddled my calculations, gas expansion from 20C to ~150C (caramelization temperature) will increase the gas volume by under 50%.