Is my illustration of this configuration good enough?
No.

First: The architecture you've shown in your diagram is grossly inaccurate. The UART in an ARM system, like the Raspberry Pi, is not connected directly to memory. It is typically connected to a peripheral bus (e.g, APB1 or APB2), which is separate from the memory bus.

Second:


  I could easily demo direct root shell if I open the serial UART connection and connect with minicom, then I got a root shell without a password with "the humble shell" hush on busybox/barebox using uClinux.


This behavior is mediated by the software running on the CPU. It has absolutely nothing to do with the hardware architecture, or the UART itself, and can be disabled by changing a configuration file.