Welcome to the forum! Your Minit looks great, what a fantastic job so far.
I replaced the interior in my Boler
a couple of years ago. I was fortunate enough to be able to use the original material as a pattern when I installed the new headliner. The corner pieces in my trailer were cut into pie shapes to fit without buckling in the material. I am not sure how I would have managed this without the originals though. Hopefully, somebody else has a solution.
As for the windows
. Since you just painted and they are out, I would install the interior first. That is what I did, for 2 reasons: first it allowed me to cut out the window holes after I installed the headliner on the wall (much easier to manage a solid piece than it is to move around a cut sheet); second, If the interior covering is behind the window frame it is a much cleaner, more finished look.
Best of luck on your project, have fun and post lots more pics!