Unfortunately, if you really feel that what you are doing has no effect and things are getting worse not better, I don't know what else there is. I am sure it's just "a phase" as we say, but if you don't think that you can work with it (or that the other kids are suffering unduly) then you may have to say goodbye. I doubt that you are "causing" anything, FS2011, rest assured. It is a fairly common thing for some kids to do.
Any chance that she is getting over stimulated? I ask because you mention that she liked the playpen, and most kids I know don't! Maybe she is needing a little more quiet-on-her-own type time?
If you do want to continue; I would give little to no attention to the behaviour. If it warrants a time-out, then calmly but firmly state "NO. We do not (whatever she did)." Then implement the consequence and give your positive attention to the child who is getting screamed at/hit, etc. Give them an extra cuddle/hug, and get them settled again.
2 is a very common age for regressing, especially if there has been any kind of change on the homefront. When she is behaving as you expect, give her some extra reassurance too.
That's all I got... lol.