I have an LG G6 that has had its screen repaired.
I'm getting a number of hardware and some software glitches. Most annoying is the microphone which intermittently doesn't work when on a call (speaker phone microphone works fine). I don't think its related to the screen repair.
Am I stuffed in terms of sending this phone back to LG to be fixed under warranty? Anyone with experience?
Most likely you are stuffed. It'll be hard for you to prove that screen repairing has not introdued issues that you are trying to get fixed via warranty.