With regards to single character versus multiple characters, the difficult part of making a single character is that it really requires the story itself to be much more engaging, because there is (in all practicality) only so much you can get by with a single character gaining without losing players’ interest. A character can only start so thin and get so fat, and the slower you make the weight gain the better your scenes in between weight gain will have to be in order to keep a player’s interest. That’s why something like The Weighting Game is so successful. With multiple characters, it is much easier to make separate, smaller, distinct plot lines that tell a weight gain story but don’t overstay their welcome. D.I.E.T does a really good job of this.
One way to potentially get around this is as @Krodmandoon has suggested - build a single character but flesh out different paths for that character as you go. Then you can focus on a single character, but you can have different plot lines for that character and tell different stories. LifeBook wasn’t fully seen to completion and isn’t a Visual Novel, but it is a good example of having a single character but different branching paths you can take with that character.