AS Java Architecture & Dual Stack

What decides whether an SAP NetWeaver AS system can run ABAP programs, Java programs, or both?

An email (SMTP) request and a P4 request both hit a dual-stack SAP NetWeaver AS. Which component receives them, and which stack processes each?

In a dual-stack system, all ABAP work processes are currently busy and a new request arrives. What happens to it and which component handles that?

In a multi-instance AS ABAP+Java system, what provides the enqueue (lock) service, and what is the alternative to designating a central instance?

A browser request enters a multi-instance dual-stack system. What routing decision does the SAP Web Dispatcher make versus the ICM?

Does a SAP GUI user reach the system through the same ICM/Web Dispatcher path as a browser user?

Is the ICM part of the SAP kernel dispatcher process, or a separate process, and what starts it?

In a dual-stack system, which component actually executes JavaServer Pages and servlets — the ABAP work processes or something else?

An AS Java system has one instance that serves no application requests but is mandatory for the cluster. What is it and what two servers does it run?

In a dual-stack system, are ABAP and Java application data kept in separate databases?