I seem to have very square feet. My shoe length size is mens 8 to 8.5 but I often end up going a size in shoes just so my toes don't feel cramped. I bought some Nordicas in Mondo 26.5. They appeared to fit the toes touching criteria in the shop. Unfortunately they have a Last ( not sure where this term comes from) of 100 mm; my toes were miserable. I though it was because the boot were too short, but as you say, they were too narrow. I've tried on boots like the Head Edge series and Atomics with Live Fit that go up to 104-106 mm but they seem too loose in the ankle.
I'm not sure if it equates to "high volume" but I think I have a high instep. The high instep with skinny ankles seems to make it hard to lock the heel in without cramping the instep. In most boots I can feel my ankle bones punching through the lining onto the shell. Finally my lower leg is thin but I have large calves from martial arts. I've tried liners, foot beds, pads and had them punched.
My solution, which has worked surprisingly well, was to get an extra size boot, high volume foot bed and thick socks. I know I'm breaking all the rules here but at least this lets me ski all day in relative comfort. I know I'm giving up some control, but fitting options are so limited. To be honest, I'm not sure how much I'm loosing out on at my level of skiing, but I'm sure the right fitting boots would help.
I also have a wide forfoot, skinny ankle, and high instep. I have pretty normal calves. I like the high volume footbed for you. Is it custom or off the shelf? Also have you tried an after market liner, like a foam injected or hear molded like an Intuition? I like my Fischer Vacuum 130 with an Intuition Powerwrap and a very good footbed made by SkiinginJackson. This will be my third iteration of this boot setup, and I think it will be close to perfect on the first fitting. Before the Fischers I used a Nordica Speedmachine 130 with an Intuition and high quality footbed for three years and loved that setup.
Good Luck with getting it worked out, it really helps.