In Vertabelo there is one diagram for one model. If you want to divide your models into separated areas "subject area" can help. I think you omit one important functionality related to subject areas. I mean "Shortcuts". Tables, Views and References may have many graphical representations on diagram. So you can have a table visible in two different subject areas that doesn't need to be close to each other.

Here is the short video tutorial about managing large diagrams in Vertabelo  https://www.youtube.com/watch?v=a79nEA0FN1M&list=P...

The design decision in this matter wasn't simple. It is a trade-off between full abstraction (parts of model + diagrams) and simplicity. In Vertabelo we generally stand up on simplicity, this way some features known from big desktop tools are not available.

It is on production. You can find the option "Primary key name" in Additional properties tab. See screenshot:

It is on production.

After a discussion in Vertabelo team we don't see the way how to support NoSQL database in our product. So I close the topic.