Tanks, please use buffs

[Lilseph]

Holy Spirit consumes a massive amount of pld's mp and can proc 3-4 times before mp gone.

5 times. And it's one of the strongest skills and should be used. If you're in Shield Oath, that recovers your guage, making the MP restoration thing a joke. If you're on Sword Oath (Which typically happens on bosses), you fill your guage steadily. Your MP should be full by the time Requiescat is ready again.

Sheltron is not a free use at will but depends on your oath gauge and kills half of it when used.

Excuses. If you're doing big pulls, the gauge should fill faster, you even can use Bulwark to fill it even faster.

Requiescat's potency decreases with MP (mp is the thing you're burning with Holy Spirit).

It doesn't decrease. If you use it at 80%+, you gain a temporal buff for all the Holy Spirits/Clemencies you cast for the duration of said buff.

Hallowed is more an emergency utility than a feature to use when available, but your suggested use is good if you're confident your party is good too. I'll not do it with PUGs.

Wrong, as other pointed out, this should be used on big pulls. It's not an emergency cooldown, it's an actual cooldown that reduces the damage you take to 0. For 10 seconds! It should be used, for instance, on the very first big pull on any dungeon, that way you'll get it available again later on for another big pull.