Nikita_Sadkov {l Wrote}:1. "Fair use" doesn't allow licensing screenshot under CC-BY
Why do you say this? All licensing does is grant permission to other people to redistribute your work, an activity which is otherwise forbidden. If fair use "doesn't allow licensing", it doesn't allow
you to distribute the work, either. Or in other words, you seem to be imagining some kind of fair use system which doesn't work.
Nikita_Sadkov {l Wrote}:2. "Fair use" is not allowed at commons.wikimedia.org
I don't know if this is true, but it's only marginally relevant. I'm talking generally here.
Nikita_Sadkov {l Wrote}:Again, derivative works cannot be licensed under CC-BY-SA, unless original work is CC-BY-SA.
Firstly, you can license your works however you want to.
If the license doesn't make it legally distributable, e.g. the GNU GPL on a program with no source code available, that doesn't mean you can't do it. It just means doing so is pointless.
Secondly, I think you are misinterpreting copyleft provisions such as the one in CC BY-SA on two counts. First, copyleft prevents re-licensing from the copyleft license to another, not from another license to the copyleft license. Second, copyleft licenses don't require that every little part be under the copyleft license. They vary a bit, but in the case of CC BY-SA 4.0, for example, the relevant text is:
b. ShareAlike.
In addition to the conditions in Section 3(a), if You Share Adapted Material You produce, the following conditions also apply.
1. The Adapter’s License You apply must be a Creative Commons license with the same License Elements, this version or later, or a BY-SA Compatible License.
2. You must include the text of, or the URI or hyperlink to, the Adapter's License You apply. You may satisfy this condition in any reasonable manner based on the medium, means, and context in which You Share Adapted Material.
3. You may not offer or impose any additional or different terms or conditions on, or apply any Effective Technological Measures to, Adapted Material that restrict exercise of the rights granted under the Adapter's License You apply.
None of this text stipulates what licenses
parts of the work must use. They could be under CC BY, or in the public domain... or copyrighted, and fair use. CC BY-SA puts no restrictions on this, as long as the whole work remains under the same license, or a license that Creative Commons has deemed to be "BY-SA Compatible" (currently, the Free Art License version 1.3). The only caveat is that if you rely on fair use for your work to be legally distributable, people in jurisdictions that don't recognize the use as "fair" will not be able to distribute the work. This is true regardless of what license you use.