Re: Atlas Bipod.
Well, with Kasey's rail, you can use a stud as one of the screws. So you would still be able to attach a sling to the stud, have your rail and Atlas, and be GTG. ETA: You may even be able to do it with both, I don't know. I just tried it with one.
Can't help you with the off hand shooting or off a backpack issue.
I will be interested to see how you make it work if you customize your Atlas.