Take information from Wikipedia and fan pages

There's nothing wrong with adding releases based on Wikipedia data, except for the fact that much of it is generally low quality by MB standards - and most editors/voters would discourage you from doing so. If you can't at least add a full tracklist with titles, times and release date/country information, your metadata probably isn't complete enough; and better to wait until a better source is found or someone who actually owns the release adds it.

You should take info from your own releases, from official websites and/or scans instead.

By scans he means actual scans of the album covers. Not all of us are so strict, but yeah, if you don't have the stuff you're adding or a trustable second-hand source for it, try to always provide several sources that agree with you. That doesn't make something sure, but it at least makes it more likely to be correct.

True; I've dealt with this a lot, especially with older, pre-Internet releases by obscure artists. The harder the info is to find, generally the worse off you'll be adding the info to MB. I recently considered adding an obscure Japanese maxi single I heard once; information on it is scant (I got it off a now-defunct fan site) and lost all the files it came with, so I held off due to lack of info.

The disc title was "守りたいから" by an artist known as ミュウ・ファイブ. I can tell you now you won't be able to find anything on it, especially considering your location.

I also think that there is a gray (blackish) area here. The Wikipedia community and editors, accept that their inserted information is CC-BY-SA, and when we copy information from wikipedia we put it into a complete public domain, without any restriction, so IMO, the wikipedia user's (who entered that information) rights are now not protected.
But as I said its a rather gray area, because this is not an information that you can rephrase or rewrite in another form. This is the actual meta-data (aka not the data itself, but the data about data).
So, IMO, its not the Musicbrainz doing wrong, but wikipedia is. They should have a separate public (or more compatible) license for meta-data.

I don't know the law part, but this is definitely how I feel and how it should be.