I believe that they made Watcher's Keep to be easier when your level is low and harder for when it's high, so I think you should be able to pull it off. Also, you can leave the Keep at practically any time and return later to continue where you left off, so that's no problem either. I'm not sure what happens if you leave halfway and return after you made the transition from SoA to ToB, but I don't think even that will cause major problems.
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