Heatmolding Question

Id think thats a shell issue and not liner. I dont think stretching it will fix anything. Im sure someone will chime in with more fitting experience but I had similar issues and after some shell work everything was gravy.

If they are brand new boots...they should feel a little tight...since they will pack out when you ride them....heat molding will essentially speed that process up.

By mid foot, do you mean the side, or the top? If its the widest part of your foot, get the shell punched out a little bit to give you more wiggle room. If it's the top, you shouldn't be in a 3 piece boot. But if it is the top, ski a few more times, if it's still tight, they can grind the boot board down a little bit to create more vertical room, but then you might run into heel lift issues by being further down into the shell.

^He has some points.
But before you start working on your shell, remold the liner with extra padding (on your foot) where it feels like it's too tight. If that doesn't work, go to a good bootfitter, tell him what the problem is and he/she will fix it. It's pretty much impossible to come up with a solution without seeing your foot but step one (if the boot almost fits good and has intuition liners) is always to remold it with padding (should have been done first time around if you thought it was a bit tight somewhere).