For classical releases, should the writer be credited if he/she only appears in the title?
I’ve found some releases that do credit the composer in the release artist field
Sings Wagner
Joan Sutherland Sings Wagner
and some that don’t
Bonus question: If the album cover says “X Sings Y” or “A Plays B”, what is the correct release title?
Sings Wagner
Joan Sutherland Sings Wagner
Plays Bach
Hilary Hahn plays Bach
I believe the answer is provided by the Classical Style Guide:
The Release title should not contain performers unless they are clearly part of the title as in “Bernstein Conducts Stravinsky.” The Release title should not contain composers except for titled releases containing their names as above, or for multi-composer releases as outlined below.
This covers your case “X sings Y” as well.
Now comes a fine print. On some releases “X sings Y” is printed in the same font, clearly indicating that X and Y are equally important (for purposes of this release). On other releases, fonts for X and Y could be very different. For example, if “X sings” is printed in a font 3 times less than “Y”, I would argue that “X sings” is not a part of the title, but just an additional information, and the title is just “Y”, that is this release is dedicated to a particular composer, and it is just a coincidence that all recordings are by the same performer. Or the other way around, if “X sings” is very prominent and “Y” rather small, than this release is all about the singer, not about the composer.
There are as many CD cover designs as designers, and obviously there is no easy rule when a part of CD cover is an important part of the title, and when it is just a side info. If one part is printed in a font 3 times larger than another, I would argue that only the first part is a title. If the font difference is very small, that is just an artistic decision of a designer which probably does not have any meaning. So there is some grey area where you just have to make a decision yourself.
Other considerations could play a role as well. Let us take some examples. Disclaimer: I do not pretend my opinion below is a Truth with a capital “T”, it is just an opinion.
Release Sings Wagner: on the CD cover we see names of the singer (Waltraud Meier), composer (Wagner) and conductor (Lorin Maazel) printed approximately in the same size. There is a word “sings” between the name of the singer and the name of the composer, so if we just put all together, it becomes “Waltraud Meier sings Wagner Lorin Maazel”. This does not sound right, so we have 3 choices: either “Sings Wagner”, or “Waltraud Meier sings Wagner”, or just “Waltraud Meier”. The second choice “Waltdaud Meier sings Wagner” sounds best to me. Besides, there is an image of the singer on the CD cover, indicating that she is important for this release. So I would choose “Waltraud Meier sings Wagner”, and Discogs agrees with me.
Another one: Celebrating Wagner. On the CD cover we see “German Brass”, “celebrating” and “Wagner”. “German Brass” and “celebrating” are in rather small font. “German Brass” is in a different colour than “celebrating Wagner”. The image features members of the orchestra, indicating that “German Brass” is rather important, even though it is printed with a small font. Besides, I would take into account that this release appears to be about an unusual performance: normally Wagner is performed by a symphonic orchestra, and here we have just a brass orchestra, so it looks like performers are actually very important for this release. As a result, I would choose “German Brass celebrating Wagner” as a release title.
Thank you for your detailed response! The “fine print” section is what I feared 
I’ve opened edits for German Brass and Waltraud Meier. Would you choose “German Brass” or “Richard Wagner; German Brass” as the release artist?
In this case I would choose both German Brass and Wagner as release artists, because both are credited on the CD cover.
As the Style Guide puts it: “In general, you should just enter the artist(s) as shown on the release”.