In a perfect setting where KYM users actually tag their shit properly, everything you've said would be possible. Broadening the use of the tagging system is almost always easier than trying to find ways around it.
For example, in situations where an image is extremely relevant to more than one entry, such as "crossover" works, parodies, or photoshop memes, the tags should refer to every work involved. Take this image for example; both the character being used, as well as the meme in reference, are tagged. In addition, a search for the artist's name or the site on which it is hosted will bring up the image. The only situation in which a user would be unable to intuitively find the image is if they were browsing the entry gallery for the meme to which the image was not uploaded to. (So in this case, a user browsing "Rikka Finger Spin" would not see this image listed, despite it being relevant to the entry. If I were to instead browse "Ellen Baker", I would see the image). In all other cases of searching, the system works fine.
As for things like multiple artists/circles, the change to the "source name" and "source link" fields on the upload page are meant to reflect this. A search for [artist name] should bring up the artist's work; the ability to then organize by circle or group seems outside of KYM's scope. In these cases, credit would instead be given to multiple artists.
The tagging system isn't perfect by any means. KYM lacks the uniformity in tagging that Boorus and similar sites possess, and our ability to enforce proper tagging is severely limited. We don't differentiate between artist, source work, and content tags, nor do we have conclusive standards for how things should be tagged. That being said, none of this should (in theory) obstruct a user's ability to find relevant content. If users tagged content properly, there would be no issue in finding it.
TL;DR Instead of a search parameter outside of tags, we should just make the tags better.