We bring 4-5 warlocks a lot (along with 2 shadow priests) and there are many times important buffs like Curses and Demo Shout get knocked off. My hunters mark is knocked off a lot in times like these to the point where I don't place it. I'm not sure if a priority system exists still or the order isn't what it should be.

If all debuffs were controllable perhaps I'd agree with you. As it is, the "strategy" is just what it was pre-tbc: have your warlocks lower their DPS by skipping their dots because the debuffs you actually want to skip (e.g. fireball dots) are uncontrollable. Or take fewer warlocks. But either way, it basically falls on only one class's shoulders (when it's not "their" problem, per se) and creates a constraint that doesn't mesh design-wise or logically with the purported reasons for the limit, nor with other constraints in the game.

But Blizzard agrees with you it seems, so the best solution at this point would just be them prioritizing what seems to be a very simple change of adding the 61+ debuffs to have identical prioritization to the level 60 debuffs. It's not perfect, since there are new things introduced, but it's at least a start since your key debuffs are common at 60 and 70.

Go shadow tailor spec. Every single warlock spec uses it. There is (or shouldn't be) no such thing as a firelock in 25 man raids.

That said, with 3 shadowpriests and 4 warlocks we have debuff slot problems. One of us warlocks goes destro always for raids. Usally, I should say.