I'm to blame for the confusion from the property explanation.
Here's the explanation, basically you get 2 free mana checks. Mana check being whenever the game makes an evaluation on how much mana is required to perform an action. So in the test case, casting greater heal you actually pass through 2 checks.
The first occurs when you actually attempt to cast the spell. You'll notice if you have 0 mana, you cannot even attempt to cast greater heal.
Once you have the spell primed and waiting there is another mana check before you actually release it to verify that you still have the mana.
If you had been mana drained between cast time and release time, and no longer had the mana, you would not be able to release the spell.
Hope that clears it up.
The charges are not rechargeable. But once the item is out of charges it won't be destroyed so it is still useful.
You may want to grab a couple of orbs and shields. They are not all the same.