Ring of Pale Order doesn't have a minimum percentage.
Being in a group with 5+ other players will reduce its effect to 0% meaning it does nothing (Besides prevent you from receiving heals from anyone else)
As Trail said, no. It is a mythic meant for very small scale or solo play. Add in the fact in PvP there's a healing reduction, and even in small scale groups it's typically regarded as a really bad idea to use. At least with Blood for Blood and Blood Frenzy there's a way for the "you cannot be healed by others" effect to fade in combat.
I typically grimace when I see anyone running this Mythic in PvP settings for the reasons that others have mentioned.
With the Battle Spirit reduction to both outgoing damage and to incoming healing, coupled with the fact that you're not always dealing damage in PvP and those are often when you need the healing the most, it's just not a strong option.
you're considered grouped in BG so Pale Order's healing drops to 0% (even back in 4v4v4.)
like the best case scenario for this thing in PVP is mucking around in IC by yourself but even then lifesteal effects are so poor because of Battle Spirit that it's a wasted mythic slot at best.
PC-NA / 1600+ CP / PVE sometimes / "Mama didn't raise no tank."