Hi,
RDBMK is not generally available at this time -- it is being made available to a limited number of customers with qualified use cases. You should work with your account manager, support, and consulting to evaluate whether or not your use case qualifies.
I can say that none of the reasons you have given are considered significant drivers for using RDBMK, but again, that is a discussion best had with your support team at Adobe.
In terms of #4, however, you can do that today and is orthogonal to the question of what MicroKernel you use -- AEM has always been able to access a database with custom services and components.
Regards,
Justin