Re: extending width of rafters for insulation
You should be able to find blue Dowboard in a 2" thickness. It would get tacked up with a few nails (they sell washer-head nails for fastening the dowboard), tape the seams in the boards with Tyvek tape or something like it. Then keeping in mind where the rafters are, hang the drywall so it goes across the rafters. You may have to cut sheets so they break exactly on the rafters. The foam is both light and rigid and won't weigh down the drywall. You should be using 5/8" sheetrock for ceilings on 2' joist centers so it won't sag.
Remove not the ancient landmark, which your fathers have set.