I'll give it a go. Can't do any structural damage.

I trimmed the extra off the closure straps, don't see why not (just leave a little spare in case you change to a larger volume insole?).

Which part do you mean? The tongue on mine has a lot of extra material, but afraid to mess with it and create a rough seam or anything.

If you get them from a physical shop that is a Bont dealer, they should have a "toaster oven" that heats them properly.