Two things.
1 – when you send the sequence (or a clip, or a series of clips) over to After Effects, you will create an .aep file which is an After Effects Project file. In order to edit it again, you merely have to right click ‘edit original’ and you’re back in AE. Actually, if both Premiere Pro and the accompanying AE comp are open if you make a change in AE, it will automagically show up in Premiere Pro.
2 – Have you tried the “ultra Keyer” in Premiere Pro? It is remarkably good and there are some broadcast shows that are using it with good results.