Using a mic with day pro for Mac

I am currently mixing with:
mac and kb cover on the keys
griffin splitter cable
I use a 1/8 to RCA from griffin splitter straight to Speaker

I know that I am getting mono sound but either way I’m using one main speaker so it doesn’t make a difference.
Question #1 - Is stereo sound only possible with 2 or more speakers?
Question #2 - Other than getting mono sound, is the audio quality compromised in any way when using the Griffin splitter?

The main Question :
If I use a usb mic will the program recognize it so i can control it with day pro’s gain, pitch and echo and will the mic signal only be sent to the main out and NOT to the headphones
Thank you very much
the mic I have in mind is samson XPD1
I had a mic that went straight to the speaker but cause the speaker was set on line as opposed to mic the red limit LED kept flashing, so I’m hoping this will be an alternative to getting a mixer