So here are a couple questions:
1. Are there any good ways to figure out original authors of sprites taken from other mods?
(Looking through various forum threads and mod credits is sometimes not enough)
Sprites are the hardest to credit because of the changes they go through. It can be easier if you credit instead the specific mod craft, units, weapons, etc.
2. What is general etiquette about using other peoples work? Is crediting enough or should I ask for specific permissions.
If there's a open source license (Creative Commons, GNU GPL, other) then you don't need to ask for permission (it's always nice to drop a line to the author though).
If there's no license, then by default is that work is copyrighted and you should ask for permissions.
3. Are there any authors, other than Hobbes, who specifically don't want their work (or parts of it) to be used?
(I would have to check my mod for other things to remove)
That was the past before I started assigning open source licenses.
Regarding XCOM Terrain Pack
anyone can use it as much as you want, just remember to credit and to make your adaptations also available to others (as per the
Shared Alike condition of its license).
Regarding Terminator: Tech-Comm,
don't use any of its assets since it's in development (the
No Derivatives condition of its license).
4. Are there any restrictions or copyright issues with using artworks or concept art for research/briefing pictures?
(For example new xcom games concept art or random stuff from the Internet)
Everything available in the internet has copyright in one way or another.
If it has an open source license, then you're good as long as you follow the license's terms.
If it does not have it, then either you ask the author for permission, or you risk at one point to get a 'cease and desist' letter.