Lemme see...
Unsure if you can dynamically change mana cost.
You can subtract effect with a stat.
FullHeal gone? Bleh.
Fireball? Not any better than smite.
Healball? Eh. Lag would make this more eh than Ampul.
Honestly, I'd rather have proper heal targetting... except I don't dare be the one to code that, it sounds irritating. (Well, actually, only the part of dynamically getting targets on the user's cursor is hard... but I digress)
Interesting, I did not think about that fact that no other spells have dynamic mana costs. See, this is why I ask these things!
As far as some of your concerns,
I have been a stark protector of Fullheal in the past, but over the last half a year I have fallen out of love with it. I don't believe it HAS to be removed or that it's too strong, I just see it's value at being very low. It's long warm up makes it hard to land efficiently as casting to early and most of the heal is wasted on your target not being hurt to much, where as casting it to late and your target dies before it goes off. It has a place, but I bet something a lot cooler could be created.
As far as HolyFlame, I was trying to move Cleric away from all autoaimed spells, and although HolyFlame cast with the default Cleric build would be no different then Smite damage wise, if you are wanting to switch up to a damage build either through attributes or InnerFire you will do 2 to 3 times the damage of smite with it. So not quite the same, but that was the intention.
As far as HealingOrb, we see Wizards tossing Fireballs and Iceballs and wrecking classes all the time so I don't see why having a projectile that heals would be so much worse!
It would be fun to have a long range aimed healing shot IMO
As far as proper heal targetting, creating a skill that would direct all heals to a designated target would greatly increase the quality of life and power of the current Cleric, but I would still love to see a Cleric that has more skill and a little more option to their play style in hopes that we would see more Clerics in the game.