Are there regular trains from Okayama to Uno?Yes, there are regular trains. The confusing thing is that the direct trains only run in the early morning and again after 3pm. But if you aim for an early start, there are two direct trains at 5:49 and 7:40. If you plan to leave later than that then you'll need to transfer along the way.
Is it better to stay in Okayama or Kurashiki instead of Osaka in order to get an early train?
You will be fine staying in Osaka. You can easily catch an early shinkansen and be in Uno at 8:23, however the next ferry isn't until 9:22.