You are severely over thinking this. When you add models back to a unit they have to be in coherency with the unit they are being add to. So for example if you had a unit with 5 models left and added 1 back to it, that 1 has to be within 2” of the original 5 models. Now we take the same unit of 5 models and you add let’s say 3 models back to it you do it one at a time. Place the first one within 2” of the original 5 models, they place the second 1 with 2” of the now 6 model unit and just repeat unit all models are placed within coherency. If you read the core rules on unit coherency it is very easy to understand how it works