Isn't is just a timed thing?  Wait a few hours and it's perfectly safe again?  I notice the lot debugger has a reset voodoo uses thingy.
The only time I've had it backfire was then I tried using it on the voodoo priest.  He wasn't happy.
Not a clue. It shows you how many uses would be reset, so you can just not touch that button and see if it ever does.
I've done it and it seems to be a timed thing : roughly each use of the Voodoo Doll takes 24 hours (I did not look exactly, let say between 24 and 36 hours) to disappear. From limited testing I'd say : 
-no uses : No backfire
-a few uses : little chance of backfire
-more uses (3+) : more chances of backfire
Someone who can look in the code could tell exactly how it works....