Hi, thanks for your comment! You bring up a great point about not having any backgrounds for certain character sprites - I did forget to mention that it's easier to find modern-day backgrounds or those featuring Japanese school settings compared to, say, Neverland, feudal Japan or anything of the Victorian era. It's one of the things that causes developers to not use specific assets or write specific stories as well, even if they could.
From developer to developer, though, I can try to help you with your issue in Point (1). You don't actually have to create a special credits page to credit the artists or sound creators - you can just edit the About page in Ren'Py, if you're using that as an engine. You can find it in the screens.rpy file and edit it:
Alternatively, you can write the credit module in your text once the game ends, in your script.rpy file itself:
I can't speak for other creators out there, but you're probably going to have a difficult time finding free assets you can use if you don't find a way to credit them, especially if you intend to sell your game. Free assets can take hours, days or weeks to make, so the lowest bar of use is to credit the creator of the free source you're using. This also extends to sound files and background music.
As a developer, we can potentially get into trouble with customers who've bought our game and decide to report us for not crediting our sources, once they find out we're using free assets instead of purchased ones since they're paying us in the first place. I've seen some pretty... vocal people on the Internet. Having a credits page tends to help us as developers quite a bit.
Point (4) appears to be specific to my assets, so I guess I can only answer for myself. I tend to put extra information within the metadata - this is because developers specifically looking to create commercial games to sell will most likely check the metadata or extended information due to legalities. Most prolific commercial game creators tend to have a dedicated team for scripting, timing, QC as well as art, animation, sound, voice-overs and music, and these are usually paid assets. When I mention my stuff is commercial-free, it simply means that I don't accept payment for them, and that I'll put out only a standard set of free stuff and modifications are for the team to make, not me. While I do allow them to be used commercially, it'll absolve me from further involvement. Legal use is always in the metadata/further information area. It's neater-looking, more professional, and still within the same page.
If you don't know anything, it's not a bad idea to ask in the comments. Hope that helps! Cheers~