Hey rabidgerry, MP1 questions probably best in the MP1 section as more members are likely to see them there and respond etc.
The master vol doesn't change the tone (although at low volumes you'll hear less bass and treb (ala fletcher munchkin loudness curves)). The idea is you set the master for each patch so when you change from one to another it's set to the overall volume you want e.g. rhythm patch to lead patch. so yes you can (and should) use it for level matching
It's best to set/adjust the master levels at stage volume, tweak them at rehearsals until they are what you want. I find the volume jump/boost needs to be more when playing with another guitarist to get your lead above their rhythm.
I now use MP2 and assign a midi continuous control pedal to the master vol (unfortunately no CC for MP1) so I can adjust as I play but then I've totally changed from when I used MP1 in a band with 2 guitars. Back then I had all my patches set to the volume change I wanted so changing from rhytm to lead was just select the right patch and I typically worked with 10 patches to cover everything so I didn't need to change banks. These days I have a clean patch, a much less distorted rhythm patch (which I also use for most solos (just turn up vol with CC pedal) and my old rhythm patch which is more over the top distorted lead sound.
Also each patch can have different (or the same) FX settings so if you have a midi rack FX unit you can change it's patch with the same button press. I use the same basic slight delay and reverb for rhythm patches and maybe a bigger delay but similar reverb for lead, but sky's the limit, you can concoct many variations
Remember though different FX patches will influence overall volume to some extent, takes a while to get just right, but then you don't need to touch it.
Like you I'm starting to worry a bit about the age of my MP2, it's still going strong but it's old. I bought another as a spare (and want to get more) which lives in my recording rack. But for a gig I feel better if I have another in the live rack just in case. That said, my live MP2 has never missed a beat (though it's not fond of gereators and sometime craps out, display goes all hieroglyphics and needs a reboot to get going again, very annoying mid song/solo, and sometimes when this happens it looses the CC vol pedal assignment, and I have to remember how to set it again)