Is there a way to turn off the "iCloud Drive isn't compatible with Mavericks" errors?
Guesswork, but the bad news is you should have switched it off before reverting, if at all possible.  

Looking through the various QAs at Apple forums would tend to lead to the conclusion that once you have upgraded to iCloud Drive it cannot be reverted. You may be able to switch it off from https://icloud.com but I can't test as I have never enabled it on my machines.

Apple have never been too concerned about people wanting to rewind.

General iCloud Drive info from Apple doesn't cover it, nor does the switching off iCloud Drive FAQ