If I read point #3 correctly it sounds like there are at least two ways of loading sprite sheets:
1) Load the completed sprite sheets at the very beginning.
2) Load individual images at first, but then the game patches them together into a sprite sheet.
Hopefully I got that right! If not I'm sure you or someone else will be quick to correct me. I just recently started to pay attention to the sprite sheet concept in game development. I had downloaded this game on Steam and the art style wasn't to my liking. No biggie, I thought, I'll just find the graphics files and change them out with my own. That's when I saw that all the graphics were consolidated into sprite sheets and that got me to wondering why they are used in the first place.