Germanic law as largely inherited from Roman law is an older view I think. As was seeing Common Law beginning with the Normans and somehow being a refinement of Roman-derived systems.
The most influential scholar reforming this view was Patrick Wormald. He roots Common Law origins in Anglo-Saxons, who took their views from Common Germanic customs. To get the flavor of what was common to Germanic groups, look up Thing (Moot, Althing, etc).
The interesting point about the origins of Anglo-Saxon law-codes is that - like many other aspects of their culture - they were quite removed from Roman influence versus say the Franks and other groups on the continent. However, this would be somewhat true for later Scandinavian systems. And, you are also immediately reminded that the codification of customs into written laws had more to do with the Church generally than with the Roman system generally (the Church is a relatively late entry into the Roman system).
Wormald is heavy-going stud,but you can look at summary sources including Wiki and get the basics: https://en.wikipedia.org/wiki/Thing_(assembly) https://en.wikipedia.org/wiki/Ancient_Germanic_law https://www.britannica.com/topic/Germanic-law
Though I'm just a ways along in my reading, it seems like at some point a clearer historical view of Germanic culture in history was developing right up into the beginnings of WWII but received a huge setback due to the abuse of Germanic themes by German leadership at the time. Common Law is a great contribution to the world, and Germany is just one of many "Germanic" countries. By all appearances, The Thing, Germanic myth or religion including Wodanism, and the body of literature including the Edda(s) Beowulf and the (related if not Germanic) Kalevala - as well as the system of runes (Futhark, Futhorc) are all part of a central culture originally centered around the North Sea and Baltic coasts (originating in maybe what is now Southern Sweden and Denmark, but spreading into the Continent and the Isles).
I think suggesting that the Common Law and Napoleanic or even Roman law are just indistinguishable parts of a synthesis misses discovering the relative and unique advantages and disadvantages that come from and are tied to the cultures originating, adopting and refining these different systems.