You can without any concerns plug a stereo cable into an unbalanced mono jack; as long as you only send mono signals (i.e. L and R channels equal) it will work just fine, except for the usually -3 dB pan law and possible short-cicuiting of the ring connector; the latter is however unlikely and not harmful for line connections. (It can be harmful for e.g. headphones connections, which you shouldn’t be using.)
To be clear: ONE input of the UR22 will be ONE signal only. For stereo you need two.
In your case you would need one rca to 1/4 inch for each of the two stereo channels, going to each of the two line inputs.
The “mix knob” has to do with what the headphones play back, so it makes no difference as far as speakers go, only the headphone output (if I remember correctly - check the manual).
In Cubase you’ll need to either set up a stereo audio track with inputs 1/2 both connected to the line inputs you’re using, or you’ll need two mono audio tracks one for input 1 and the other input 2 where 1 is panned left and the other right - and in both of those cases you’ll get actual true stereo.
The mix knob chooses whether the output signal comes from the Daw or directly from the inputs or a combination of both, I am aware of the possibility of running the tracks through a daw and panning them there but I hoped I could pass a stereo signal without the conversion to digital and back to analog, thank you for the help.