Thank you for the good questions @queal101!
The consistency is that of a milkshake or a little less thick. Of course if you add more water then recommended, it becomes more watery. Some like this, others like it with less water. Both are fine ways to prepare it.
The sugar comes from the carbohydrate sources and there is no sucralose ('simple' sugar) in Queal. It's from the maltodextrin (from corn) and isomaltulose (red beets). But a part of that has to be classified as sugar. Because the Athletic version has more protein, the total carbohydrates (and thus sugars) content is lower.
The sweetness is the same, this comes from the natural flavouring that is the same in all variations.
Yes, it's perfectly fine to have a smaller meal (e.g. for breakfast) and just adjust the water proportionally.