1) There are a few differences - you can just read the README and Help files of each to find the nuances. Basically, code keeper is designed with the idea of using macros, plugins, and code taping (patching in entire file contents from one dba script to another). DarkEDIT is more straight forward. It does allow similar code insertion, has definable types and globals, and a few other features.
I think code keeper is great, but I had gotten so used to DarkEDIT that I had no real reason to use it. DarkEDIT is a nice step up from the default IDE that DarkBASIC is packaged with.
2 & 3 I don't think either of them are worked on any more and haven't been for some time. Maybe Code Keeper is, I haven't looked for it so I don't know, but my guess is because DarkBASIC Classic is quite a bit antiquated and has essentially been replaced by DarkBASIC Pro, it's not being worked on.
I think TDK_Man had been working on an editor for DarkBASIC. It's on the WIP (work in progress) board. If you contact him directly, I'm sure he can give you more info on it.
Enjoy your day.