You have a pluralization error in the description, then. If you can only spend one hit die, then that solves the inconsistency issue pretty well.
It was not intended as such. The thing is, I could figure out the correct spell to cast in any given situation, but it would require calculating out each equation independently in my head in order to compare them. There is still a correct answer for every situation, but that answer changes based on a bunch of known variables.
Why would I want to play a game that asks me to do a bunch of extra math, after I've already decided that I want to cast a healing spell? Deciding to cast a healing spell (as opposed to casting a buff spell, attack, or some other option) was already the interesting choice to be made for the round. You've added a bunch of unnecessary complexity that slows the game down to no real benefit.
Somewhat fair. When designing these spells I wanted each one to be good in a different situation. Here is how I break it down in my own head.
Healing Word (Mass): I don't want to waste my Action.
Cure Wounds: I want to heal a lot of damage fast, but at a cost.
Litany of healing: Heals a lot but is much safer to use outside of combat.
Mass Cure Wounds: I want to heal the entire party and don't care about the costs...
Heal: I want reliable healing without a cost.
Mass Heal: Best healing spell.