I cannot speak to everything that you are asking above. But one thing I can clarify, is Tableau desktop will not make exclusively 1 to 1 releationship joins. Tableau Desktop can join with a many to one or one to many relationship, depending on what join type is selected. Thought the results of joining on a null might not be what you are looking for. Tableau allows as much as possible when it comes to connecting to the data as we do not know every use case out there. I am sure there is a scenario where someone wants to join to disparate data sources on a blank columnn. The precise logic behind this is a little bit beyond my scope of knowlege.
One suggestion I would make is that when you are creating joins to fully research the different types to understand the results that are being returned more clearly.
Hope this helps!