I think you'll find with a stem mount the Garmin will be a whole lot more protected. I've had some fairly major 'yard sale' type spills and it's stayed connected OK. You could be on the money with trying to pack out the mount to raise it a little - have another look first as the Garmin sits higher than the mount, giving it a bit of clearance, and I've not had any problems getting my unit to clear the bar clamps - on all of my bikes, which have a combination of Race Face, Easton and Giant Connect stems, all on 31.8 bars.
As to the AYUPs, I run mine horizonally and no issues with the lead, I think it actually makes management a bit easier. I can run it in under the stem, up through the gap between bars and mount and then plug it in.
You may get a little slippage between mount and bars, (especially if using carbon bars), this can be combatted with a dob of Silastic or a little cut up inner tube between mount and bars, just to give it a little 'stiction'.