So Beans, I wanted to double-check my answers with Andy since I do not speak for, nor am I an employee of Mesa. I do get some inside looks, but the answers below are my words. Andy felt they were accurate.
Your first question is tricky one from a safety and legal liability perspective. As you know, if someone gets hurt doing something they weren't explicitly warned against in the simplest and most unambiguous terms, there is an opening for a lawsuit. So, because of safety regulations (which have to be satisfied both in the US and internationally for products like this), and the need to address the lowest possible common denominator when it comes to techncial skill/knowledge/experience, when the top of the amp needs to be removed, everything inside the amp becomes non-user serviceable. This is because if the amp is powered up, seriously hazardous voltages are present close to the tubes. Additionally, the mains fuse is not intended to be replaced, it is a safety device that opens upon a catastrophic fault and replacing it could lead to additional damage or dangerous conditions.
The bottom line here is that Mesa is doing everything possible to ensure user safety, and as you know with your techncial background, many folks simply do not have the knowledge, training, experience and tools to safely work on things like this. Yet lo and behold as I'm sure you've seen numerous times, many will try anyway assuming a screwdriver makes them qualified to open up an amplifier and remove and replace parts. As you know this can lead to more serious consequences than just an amp that's not working, but most frequently ends up with a repair that is far beyond what the original problem would have required.
Switching the impedance selector with the power on won't hurt anything, nor will plugging/unplugging a speaker provided both ends are SpeakOn. For me as matter of best practice I always recommend first ensuring main power is off, mute is on, the masters are fully down and the impedance selector is in the correct position for the load you plan to connect,
then connecting the speaker load, double-checking all conections, and finally double-checking the impedance selector position (which can easily be bumped when connecting the cables) before turning on main power.
I know that sounds kind of aviation checklist-like, but as you know my background is avaition and procedures and protocols followed consistently tend to consistently produce the desired outcomes with no one getting hurt and all equipment still in working order when the job is done.
