There is indeed a lack of Pull Requests, which would make it vastly easier to contribute. It would be interesting if someone can figure out how easy it is to implement that into the AUR.
Typically one would just make a patch file and link to it in a comment. Even if the owner does not apply it themselves others can choose to.
4. upstream software author is difficult to directly contribute to the AUR packaging contribution. For a great number of upstream software author, they are willing to participate in downstream redistribution process. Especially for those young software. But the current AUR workflow block their way to contribute on downstream stuff.
An upstream maintainer can just ask to be a co-maintainer of the downstream AUR package. If a maintainer does not respond, that is already a reason to orphan the package. Thus the only problematic situation I could think of is where a maintainer does communicate but is not willing to fix an issue with their package, which I have never seen myself. I think it's good not to overcomplicate the system. I don't think the same software should be provided by multiple PKGBUILDs or maintainers. All software should just be packaged correctly on the AUR.