Because the water is bottom loading, is all the water used up or is there a small amount remaining at the bottom of the bottle?

Does not get all the water, usually 1 to 1.5 inches of water left in the bottom. The cap design is not the greatest in the world, and they have stopped making bottles with the extra lip at the top, so it is not secure at all on those new bottles. Go with the top loader, and eliminate having these issues.

There is very little water left in the bottom of the bottle when the light comes in. I would say less than a 1/4 cup.

The dispenser pump does a good job of drawing all of the water from the bottle. However, some bottle shapes (some without a flat bottom) may allow a few ounces of water to remain.