I'm not sure exactly as I haven't read it. But generally, a light novel often involves a unique story, perhaps in this one it could be about a vampire (kyuuketsuki) and their daily life including an afternoon nap (ohirune).
It could mean a lot of things. Maybe it's a name of a special being in the fanfic's version of the Naruto world. It might be an elf - like creature that has some kind of power that can't be tamed or understood, which is what 'yaserarenai' could imply.
Yes, it can be used. You could incorporate 'elf san wa yaserarenai' as a unique concept in a Naruto fanfic. For example, if you have a character in the Naruto world who is from a different dimension or has a strange power, you can name it 'elf san wa yaserarenai' and build a story around how it affects the Naruto universe, like how the characters interact with this new element and how it changes the power dynamics.
Some local libraries that have a good collection of international or Japanese literature might also have it. You can also search in some anime or manga - related forums. Sometimes fans share information about where to find rare or not - so - popular light novels like this one.
I have no idea about this. It might be a niche or uncommon type of comic that I haven't come across. You might have better luck searching on dedicated comic-related websites or platforms.