Nullvoid, Dimension12, Ledgerdomain branches off exactly as many times as many times the ben Dimension in a way that each branch have the all branches align together with perfectly the same branch of other dimension that they deal with. The idea that they are within same universe and timeline is Accurate and idea that they are branching off independent of each other is just headcanon and takes more assumptions and contradicts many statements about these Dimensions being inside universe.
Legerdemain, Anur system, Dimension12, nullvoid these all Dimensions are just either homeworld of some species of the universe, or jail for the species of the universe, that's idea alone looks enough justifiable. And we have statements is just iron the things.
Also your explaination seems like you already conceeded tbh.
This is the way they branch, in parallel and together and in a way that each branch of ben Dimension aligns with each branch of other Dimensions perfectly (the one they deal with not with a branch that has nothing to do with their branch), they're not outside of timestream but rather inside and branching off together.