1. Install K2 v2.10 dev from github.com/getk2/k2/archive/master.zip - it should resolve any issues. If not, delete the folder /media/k2/assets/ and re-install K2.
2. I don't see how this is relevant to K2. If you don't want K2 to augment signups (not logins), disable it in K2's Settings (under Advanced >> "Enable K2 User Profile" - set to "No").
3. First off, these issues are not in K2. Some are warnings from a third-party plugin (not errors) and then it's Additional Categories which is waaaay old and unmaintained. I highly recommend you get this: www.web357.com/joomla-extensions/multiple-categories-for-k2 (it can even import associations done in Additional Categories). It's brand new and actively maintained.