I've never seen it "available" because they cast it every time it's cooled down.  The only way it would be available would be if there was no one in range that had taken damage in which case there would be no need to heal.  So, while it's possible that is the case there is no way I can think of to test it since they will always auto-heal faster than you can click (if it's not cooled down).

If you are referring to locking them on to you, I tried that.  I selected them and right-clicked myself but they still healed another minion instead of me.

You can thwart this to a degree by positioning healer units in your army with Move commands, positioning them generally closer to your Demigod than to the frontline of your other units. Not micromanagement, yet something to do beyond a regular Assassin`s routine.

Healing Minions isn`t necessarily bad.

Oh, I wasn't aware of any such buff.  It's very possible that was what it was.

I say remove it then because as it is it generally isn't worth it to buy any minions.  I've seen some people buy the 1000g priest just so they have a little more lasting power up in melee but I don't think I've ever seen anyone get the level IV minions.

Yeah they actually do heal quite a bit. That's why the 1k gold ones are actually worth it.

I don't think the 4250 or whatever it is gold ones are, though.