A VERY interesting article on motorbikes (CMV July 2021) although the section covering German motorbikes is somewhat inaccurate.
It looks like you are mixing up two different BMW models, the R75 which is the main subject of the section and the BMW R12 that you do not mention at all, but refer to an R35 as close relation to the R75, the R75 being called its "big brother".
The R35 and the R75 are miles apart in technical design, one is a solo (usually) single-cylinder 350cc bike and the other is a 100% sidecar fitted flat-twin 750cc bike, so where is the connection other than both were BMWs.
You state that: "by the time the war started the bike (the R75) was ready for service." This is not correct, the first BMW R75 left the Munich production line in July 1941, approximately 21 months after the start of World War Two in September 1939. The BMW 750cc flat twin with or without sidecar that was available in September 1939 was the R12.