A 1/50 Possibilty, perhaps. From Gnomes and/or Elven Races- In the same way Elven Fighters start with a Potion of Extra Healing.

There should be a 100% possibility. With 1/50, there's no point to even bother.
Those potions can be found in ID:1 randomly, they are *very* common and don't give you some tremendous edge at the beginning.

This would be great. PoInvis provide great one time tactical advantage that is very helpful to thiefs - whether they need to rob shop or run from gnoll chieftain.