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In short, much of what you say is just true. This is the actual sad state of metadata, unfortunately, at least on TiVo and Musicbrainz, which Roon uses. (There are some other potential sources like Discogs but they have their own problems). On the other hand, Roon is still better IMHO than any other music players, so I live with it (and edit what is really important to me; best to do that directly on Musicbrainz so that everyone benefits, but it is a bit of a slog. You can also report issues in Metadata and sometimes they get fixed).
There are lots of discussions about it, most recently (I think):
Some specific comments from my experience:
I have never experienced this, no idea.
I don’t know if this is what you mean (and haven’t tried the workarounds you listed), but there has been a glaring issue for years with some tracks not being recognized as compositions and hence not linked to other performances. There are several older threads about it where users gathered intel. I suppose the most recent one is my more formal problem report (where I also linked some older threads) which, after some initial progress, unfortunately led to nothing. Don’t ask me why, linking performances to compositions is one of Roon’s major selling points and IMHO should just work (with some allowances for cover versions with differing titles, e.g. because it’s in a different language) and should definitely not fail because a track is missing the “composition” property.
I may have seen this maybe 5 times max. with approx. 1000 albums imported. Are you sure your file tags are in order?
Typically this is caused by not following the correct arrangement:
Never seen this. Aggravatingly incorrect credits, yes, and other issues like the very common incorrect equivalence of artists with the same name, but not this one.
Not sure what you mean, other than credits being inherently messy because there will never be an agreement on everything 
Generally, I don’t think you will be happy with Roon if you intend to spend half your remaining life on forcing it to conform to your personal taste. You won’t win. Roon has strong ideas about how things should be done, sometimes more successfully and sometimes less, but it’s best to go with the flow (and make edits here and there where it is really important). It is possible that it is just not for you.
Yes this is annoying, in particular because every user has to find out themselves, after they already created a bunch of duplicated artist stubs. I have written about this quite much on the forum. (Luckily, you can merge such accidentally created artist stubs into the main artist)
If it helps, you can remove the placeholder albums from the library again, after having used their artists, if you only added them for this purpose. But still, it is a lot of work to ensure the existence of artists in the library every time before importing something.
It could be handled better in the UI, but the fundamental reason for this is probably the architectural fact that Roon maintains your local database only for the albums/artists that were added to your library. I don’t know if it could always search online databases (like streaming services and MusicBrainz/TiVo) to find artists, but I suppose this might be limited as long as Roon Server runs on a local computer (with limited resources) and not the cloud.
This distinction between objects that are in the library and those that aren’t lead to many limitations and some weird behaviors. Much of this is summarized in this thread and in other threads linked from there:
but there are also other dispersed feature suggestions that are probably technically related such as this: