Originally Posted by
QuantumFX
I can't answer for Unyielding Sentinel, and there's some tests that I'd like to try in a quest with perma dungeon alert, or the mobs stacked on top of each other in a 3D space*, but it seems to be: so long as nothing is trying to attack you, you get the full heal. My test for this is watching the procs for Sustaining Song, and Holy Presence.
*Think the Yugoloth favor Amrath quests.