Suppose I make non-conflicting changes on a shared file on two different devices and then start syncthing to communicate those changes. Will I get all the changes or just the most recent version of the file?
You should get a conflict file created. If you happen to be using Synctrazor on Windows you will also get a GUI to help resolve the conflict. Otherwise you need to notice the files and manually resolve them. Syncthing is designed to not discard data in any situation