It's a crap assignment because even if you would end up delivering something that could be 'objectively' (by majority of laymen or professionals) judged as pure capture of reality, your client could simply decide it is "not his" kind of reality. Anything creative, is heavily subjective, who here didn't have client who would proclaim upon perfectly correct, highly realistic floor something in lieu of "I think it looks strange/fake, can you remove all the reflections and make it look super flat and ugly ? I think that's what the floor should look like".
Or my personal favorite "I don't think the mirror is showing correct reflection, shouldn't it reflect the other side of room" (Well of course, I will tell my renderer software to go back to geometry class).
This is why you have in you contract a clause that prevents this kind of argument, establishing quality that follows the common industry standard and the acceptations of limitation of technology.
It's ok to promise the best based on your existing portfolio, but not be legally beholden to unrealistic objectives.
you will always have some great thoughts on different subjects,
thank you for your sights, really interesting to read!
here is old project by agyoutcast (even they are low resolution, they both somehow look fake and cg to me despite of their appearing "cleanness", i cant really distinguish them lol)
Kamppi Chapel

for me, as total rookie of this industry, photorealistic image is one with:
-perfect model scale,
-rich GI,
-great and detailed textures,
-lens imperfections - scratches etc,
-vignetting
-floating dust,
-lot of noise in darker areas,
-lens distortion,
-CA
-one with really damaged look.
i really doubt any client would ask this damaged looking-raw-photo-like image for any value and use for. all based on personal preferences, you should ask client what do they interpret as realistic. personally, i dont find realistic these toxical-clean looking furniture magazine artworks.
for me, this project by Sergey Baskakov, was one of the most realistic archviz looks i have ever witnessed
https://www.behance.net/gallery/21833701/Noncommercial-project-House-and-nature