You seem to be after flavor on the first part - it's not really explained in the rulebooks how something that doesn't eat can still heal up when it's lost mass due to wounds. But they can, same as anything else (other than Constructs, Mindless Undead, and other exceptions to the general rule, anyway).How doe Elementals heal? Do they "regenerate" (but not the power, if you know what I mean) their mass or matter or energy? Can healing/harming spells affect them? (There seems to be something woring with that, but that's neither here nor there.)
So healing a fire elemntal should probably cause a cleric (or whatever) some damage, since he's putting his hand on PURE FIRE.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.