Do you happen to know how can I - and others - ask DA to make the creative commons under 4.0 attribution? I went through the help desk and I didn't find a way. I probably missed something. Can you help me?
There's no way to specify using the deviantArt tools, but you can always not specify a creative commons license in the tools and put your Creative Commons license in the description. This should include a link to the cc page/terms and perhaps a graphic. This goes the same for Public Domain, CC0, GPL and other possible licenses.
It can for compatibility. For example I've been asked to release code under Apache when it was GPL already. (I didn't agree to since I'm fairly fanatical about copyleft) but that's ok, plenty of project do duel license for these reasons.
Yes, that's true, though it's mainly a concern with software licenses. For artwork, there really aren't the same concepts of linking, source code, or patents, so there is a lot less room for incompatibility.