If I was building a Warlock for that group, I'd go with something like this:
You have two defenders, which is great for you. They should be able to keep most of the heat off of you. Which really means you can get into the thick of things and flank with your allies for increased accuracy. The few teleportation powers can help you with that.
You're right you don't want to push things away from your allies. So Diabolic Grasp is going to help you slide enemies, mainly toward your group.
Since you don't have a leader, Frigid darkness can easily be used as the leader nova power to help out your paladin and warden.
And Mordant Rains of Dis will help when your defenders haven't been able to lock everything down.
You have a very control heavy group, and should capitalize on that with power choices, making everyone's jobs easier.
The rest of your attention should probably go toward damage output. Armor of Agathys and Hunger of Hadar can both significantly increase your damage output. Paying a bit of extra attention to your implement to use a staff of Ruin will also give you a good bump to damage. A Quick Curse Rod in your off hand will both give you a relatively cheap bonus with dual implement, and allow you to get curses out there quicker. At paragon you should start cursing targets two at a time.
Your basic Eldritch Blast is going to be +13 vs Reflex, +16 with flank and prime shot which you should easily gain thanks to your Shadowdance armor, and +18 with single triggering of the pact boon. And you'll be doing 1d10+1d6+13 damage (2 more against bloodied target). That's not too shabby, and your accuracy will be better than anyone else. This also means you will and your encounter powers more often than others, and your daily powers don't even need to hit to be effective.
And in the skill department you will have respectable Stealth and Thievery which should both be useful in your group.
At paragon, which you are pretty close to, you can gain a significant boost to your damage with Called Shot as well.
You have two defenders, which is great for you. They should be able to keep most of the heat off of you. Which really means you can get into the thick of things and flank with your allies for increased accuracy. The few teleportation powers can help you with that.
You're right you don't want to push things away from your allies. So Diabolic Grasp is going to help you slide enemies, mainly toward your group.
Since you don't have a leader, Frigid darkness can easily be used as the leader nova power to help out your paladin and warden.
And Mordant Rains of Dis will help when your defenders haven't been able to lock everything down.
You have a very control heavy group, and should capitalize on that with power choices, making everyone's jobs easier.
The rest of your attention should probably go toward damage output. Armor of Agathys and Hunger of Hadar can both significantly increase your damage output. Paying a bit of extra attention to your implement to use a staff of Ruin will also give you a good bump to damage. A Quick Curse Rod in your off hand will both give you a relatively cheap bonus with dual implement, and allow you to get curses out there quicker. At paragon you should start cursing targets two at a time.
Your basic Eldritch Blast is going to be +13 vs Reflex, +16 with flank and prime shot which you should easily gain thanks to your Shadowdance armor, and +18 with single triggering of the pact boon. And you'll be doing 1d10+1d6+13 damage (2 more against bloodied target). That's not too shabby, and your accuracy will be better than anyone else. This also means you will and your encounter powers more often than others, and your daily powers don't even need to hit to be effective.
And in the skill department you will have respectable Stealth and Thievery which should both be useful in your group.
At paragon, which you are pretty close to, you can gain a significant boost to your damage with Called Shot as well.
