It's easy enough to turn the volume down if I don't need the alarm. It's not always easy to unplug it. Turning the volume down is the easiest thing to do. Light from the alarm clock/clock radio is minimal. It doesn't bother me. If you can sleep on a plane, you can tolerate light from the alarm clock.
For an early morning flight I never depend o just one thing. Front desk might not put your wake up call in--it does happen once in a while- the mobile alarm might fail to wake me up. II set the alarm and tune the radio to a frequency with decent reception, set my mobile phone alarm, request a wake up call from the front desk, and have my wife call around 5 minutes after the wake up call and the alarm. That way, we can talk while I am having my morning tea or coffee, just like at home.

If you unplug the alarm, I think you should plug it back in before you leave.