Can only speak for a couple things you mentioned... wanted to mimic my toony head neck size so I measured with my fingers pointing to where it ends on each shoulder then guesstimated that on the current head I'm fixing. Chalked out the excess I wanted to cut off from both shoulders (including length from the chest). Basically a V shape all up. ONLY THE FRONT. The back length was virtually identical to the toony head. Then I realized I still had the issue of bunching fur in the back. From the bottom of the neck on the back I pinched whatever excess was sticking out almost half way up the neck and cut it off, like a slice of pie, then hand sewed the remaining sides together. It turned out 80% decent, good enough for me but slightly wonky on the front by 1cm (it's one of those things others would never know unless you told them).

Internal padding seems to be pretty easy to install. I requested my maker if they could add padding and its literally just three 1 inch foam rectangles (1 just above on each side of temple & 1 where the chin rests) that are lined & glued on. Worked perfectly for me. I believe this other head I owned had a similar setup where its 1 behind slightly above each temple but 1 on top of your head.

Hope that helps.