1) Adams ultimately settled on HC-110
2) Generalizing about curve shapes based on the developing agents is an incorrect approach. Little can be said about the characteristics of a developer without looking at the concentration of developing agent(s), the concentration of preservative (Sulfite) and the concentration/type of alkali (pH, buffering). As an example, G. Haist (Kodak) created a modified version of D-76 with identical working characteristics (but less capacity) by slightly increasing the amount of Metol (Elon) and omitting the Hydroquinone entirely. D-76 is a fine grain developer very similar to D-23 in practice.
3) Microdol is said to be D-23 with a little less Metol (5g/l rather than 7.5g/l) and somewhere around 30g/l Sodium Chloride. Microdol-X contained at least one extra ingredient - an anti-silvering agent.