"Is there a way to add return pieces to quarter-round after they're already installed? I know that I would normally cut the ends at 45deg and then add another 45deg piece for the return, but I'm wondering if there's a way to do this without a 45 on the installed lengths. Right now they're just squared off."
I'm suspecting that someone started installing those quarter rounds and never finished.
That's the way they're supposed to be when installing "quarter round" which is in all probability a wood molding called "carpet strip".
On the outside corners you cut both pieces at 45 degrees.
On the inside corners you butt one end into the corner and then you do something called "coping a miter" on the piece that fits up to it.
To "cope a miter" you cut your wood molding at 45 degrees just like you were going to "45 degree" the inside corner.
But, then you use a coping saw to cut the wood off so that your "coped miter" fits right up to the butted end of the other piece.
Doing it that way will look better than if you "45 degree'd" the inside corners, and if the wood molding shrinks a bit if it dries out, you won't have any gap open up on you.
If someone has started installing your quarter rounds but never finished, you simply need to Google "Coping a miter" to learn how to finish that job.