Did you really need a warlock, leaving class balance aside? What would've you gained? Would your BrM stuck with the guild if you wouldn't have waited for a good player?
Having a good tank tag team will make a difference in any fight. 2 new tanks mid tier is never a good idea
a) learning how guild handles the mechanics
b) healers have to figure out the dmg intake and the rough squishiness level of both tanks
If you'd have stayed as you tank, then there
a) would've been 1 tank that could walk the new tank through how things are handeled
b) healers would know how to deal with 1/2 tanks
Yes, you made a mistake by changing both tanks. No, you cannot change the fact that you might slowed your progression down. All you can do is hope now that both tanks (and your healers, as I do feel that loosing a shield is more or less healer deal (though then again I can heal my shield to full on my own as a guardian) in Molkorok) shape up a bit and you don't loose people while you try to find your game again.