Always pick up adds. That's the rule, with very very few exceptions.
You're using Provoke too early. It's a safety if you lose threat, or if there's an add. There's no advantage to using it early or immediately, especially if you already have threat on the target (unlike XI). Save it for something that you lose control over, and immediately follow it up with a high-threat ability like Savage Blade or Rage of Halone.
There's also no point in using Shield Lob a second time if the targets are within melee range. Keep in mind there's a 2.5 second GCD, so don't throw a second Lob if the enemies will arrive in a shorter window than that. Start your actual threat combo.
People say to spam Flash twice, but that seems wasteful and lazy to me. 1) it cancels and refreshes the Blind effect immediately, cutting the effectiveness from 12 seconds down to 6, and 2) it bumps the enemy's resistance counter toward it up even faster. There should be no need to do that if you are switching targets in your rotation to mix in some additional threat.
To date, I have never used the MP regen ability, nor have I spammed Flash so much that I ran out of MP. The only time I do agree with Flash spams, or at least a consecutive double-Flash, are adds that come in during bosses when you're juggling 3+ enemies already. Otherwise you should have plenty of time to weave in threat on the new enemies with the normal rotations.
The bulk of your threat should be going to the target that all the DPS are on. If you're marking 1 with the expectation that they are going to kill 1, there's no reason why you should be dumping your first combo on a totally different target.
My usual combo is:
1. Target enemy, Shield Lob
(enemies should close in; if there are ranged/casters, I try to LOS pull them so they cluster together nearby)
2. Flash all of them
3. Start my combo: Fast Blade -> Savage Blade on the main target, switch to a different target for Rage of Halone. If you have Spirits Within, I just use that on the third target.
4. Return to main target, throw Fast Blade -> Savage Blade again, RoH on third target.
5. Recast Flash
6. After this point, I will just focus the main target with the full combo. Every other combo, I will throw one of the two (Savage, RoH) to the other targets (Savage one, RoH other off-target).
Mindlessly repeat while avoiding AOEs and such.
If the DPS is splitting their damage, then I will split threat in the same way, divvying up Savage and RoH between the targets taking damage.
You can switch targets to the next mark around 10% or less. Just ignore it; even if DPS peel it, they can polish off the last bit before it even gets close enough to touch them. That shouldn't happen, but it really doesn't matter if it does. Just start your threat combo on the next target.
Edit: at 22, you don't have RoH yet. Just switch to one of the non-focus targets every OTHER combo, and you should be fine, if you periodically add a Flash after 2-3 combos.
Also to add insult to injury I had to lay down there dead at the bottom of the cliff for 5 minutes.
