I believe that folks are making more of a job out of this than is necessary. I had planned to disassemble an engine for some parts and finally got a chance to photograph the technique. Here in photographs is how I remove a rear main oil seal from the engine case of, in this case, a 1978 R100/7. Notice that the bottom lip of the seal is black from oil leaking past it and, the outside edge of the seal is even with the engine case. The seal is an original factory plastic seal correct for 1978 which has probably not been replaced before this.
First photo is the seal in place: Note oil on lip and seal position in case.