When the headphones are plugged in, is there a keyboard sequence you can press to turn the external speakers off on the notebook?
No. Nothing that I can readily see or find in Google.
Other than sound coming from the 'phones, is there any feedback at all that they've been plugged in? If there is, it can be hooked.
No, there are no notifications or anything that says: "The headphones have been plugged in".
By the way, I just tried my 'phones with Edubuntu and it works as it should. So, the computer is not the culprit.