One of the advantages of having the entire branch history in front of you is that you a potential reviewer can do a diff of the entire branch starting from the very base, so that you can still get an overview of all the changes. I believe it'll be understood that the reviews will be preliminary in nature, but also that having more sharing of changes as they happen will lead to any issues with the overall design being caught earlier, before too much work has been done.

I think that documenting the design is going to be a big barrier, and will probably as you note cause its own set of problems.