Yes, I know most of this, as Graham and I are writing the book you mention (unless there is another).
We sat down two weeks ago for a week at my house (he lives in Chicago area, I live in the Denver area) and we finalized the book.
We cover about 5 pages on simson history, which was gathered from many sources and used with permission. There is an interesting new book, mainly on Simson motorcycles, a German made book "Simson Legende" that was interesting.
Much is not known, they did continue to produce weapons, although being "broken" up they were renamed and limited. The MG line evidently stayed in production. Much is not known, as the Simsons and upper management escaped, the company diversified (again) and the DDR took over for 40+ years. So most records were lost or destroyed.
The P08 machinery went to Krieghoff and was used until they made their own, then probably scrapped over time.
The family tried to 'reclaim" their property in Suhl and were either paid a small sum or disallowed (local people always liked the family, nazi officials did not, and the later DDR was probably influenced by other local gun manufacturers, but that is conjecture)
We hope to publish in the next couple of months, those details are being worked out with the publisher (s)...
Ed