Healing to max health

Not totally sure why it needs 50,000 health, but yes, I imagine that's outside the allowed bounds of the game. Normally, you would just set the entity's current health to it's max health, and it wouldn't be an issue. If this is causing problems, you could consider using absorbtion hearts... or just not giving the boss so much health? You can add built-in armor if you're worried about players defeating it to quickly. 50,000 is just really absurd. The warden is meant to be basically unkillable, and it only has, like, 500 hearts.

You could also just make your own health system. Make the entity invincible, but instead decrease some NBT tag or variable whenever it would normally take damage.

Oh, if you just want it to regen quickly, that's much easier. You could simply make a procedure that adds health, (by setting its current health to its current health plus a certain amount every tick), which would cause its health to increase at a constant rate. If you wanted to be a bit fancier, you could make it recover a percentage of its missing health each time the procedure runs. (You would instead make the amount added to its current health the result of multiplying the difference between max and current health by a given percent), which would cause it to heal proportionally faster the more hearts it's missing. 

Because the procedure will likely run every tick, if you want it to heal less than half a heart per 20th of a second, (the equivalent of recovering 10 hearts every second, which seems like a bit much), you could add an NBT tag to the entity that acts as a timer between heals; countdown the tag each tick, and when it reaches zero, reset the tag and heal the entity.