If I select 'lock device' in Android device manager, will it affect scheduled alarms? I have left my phone at my girlfriend's house - it's definitely there according to the location feature in Android device manager. 

There is an alarm set for really early in the morning, which it would be great to turn off.  If I select 'Lock device', will this have any effect on the preset, scheduled alarm set in the stock clock/alarm app?

Presumably I could definitely stop the alarm if I selected 'Erase device' but this seems like a drastic option!
No,no effect on the preset, scheduled alarm set in the stock clock/alarm app because the alarming processes are generally given in a 'Android Service', that is a background process. Lock screen will not affect a background process.