it all depends. it may end up being easier just to build an extension cab and have it rigged up to a 12" instead and making the combo pretty much be a head. and then when you just want to play at home, have it where you can just use the smaller speaker.
opening the back opens up the bass response and makes it breath more. I like the sound of open, or semi-open back better than the boominess of closed-backs.
always a good thing! the only thing bad for a tube to do is not glow.
Oh yeah! Go for a BBE Green Screamer... They're a real sleeper pedal/ diamond in the rough! Yeah the muff, may be too much for the small speaker.

sorry man...
Nah, just find a simple way to rig up the extension jack like I was saying [BUT NEVER LEAVE THE CABLE TO THE SPEAKER FROM THE AMP UNPLUGGED AND THEN TURN THE AMP ON; YOUR AMP WILL DIE] Okay?
i think your teach may appreciate it more once it's at a more manageable volume.