Time Shift limitations

Time Shift does not get consumed by skills that have no cooldowns. Time shift only loses a charge when you attempt to load a skill which is already on cooldown (i.e. it must have a cooldown to begin with). Doing so consumes the charge to instantly refresh the skill's cooldown and allow you to load it.

As for its "main use", that largely depends on playstyle. It can be used for spamming AoE abilities such as Snap Cast or Lightning Rod, or to receive a single refreshment on Final Hit / Way of the Gun, or to instantly drop to negative HP through multiple Dorcha Conversions. I've also seen individuals use Time Shift as a backup for Combo: Charging Strike when attempting to burst a boss's HP through repeated Crit resets via the Revenant and Perseus knuckles.

Does it work on the Divinity's ultimate AoE skill if you activate the buff before you transform?

AFAIK it does not... but I have not tested this. Go find out :)

I wouldn't be asking if I didn't have only 2 techniques I can use out of the ones I did unlock (The Iron Will and Defiance). 1500 weekly points limit is annoying.