Characters cannot be stunned, so to speak. You may see stars, but they don't affect your attack rating (it should, it's cool). As for mercenaries and minions, I think you'd have to test it thoroughly. Fully synergized, Stun gets +100% from Warcry and starts at 1.2 seconds - gaining .2 every level until 15 (4.0), where it gains .1 from then on. I think SLvl54 is the highest attainable, and 15.8 would be the absolute limit, so just get someone to disarm their merc and test it out.
As for Grizzly, it's still affected by the PvP penalty, no exception, AFAIK.
So, stunning an opponent has lower odds than stunning a super unique, but - once done - any stunned opponent will have a 100% chance of successful attacks triggering hit recovery? Interesting. Does this include non-physical attacks or just anything that could ordinarily cause hit recovery if otherwise successful (Fireball, Inferno, Arctic Blast, but not Poison Nova)?
It's be interesting with Arctic Blast and Inferno, since they normally can't do enough damage to trigger hit recovery, unless they're like poison damage. Stun with Frostwind and open wounds... ew.