According to Wikipedia, Modified Harvard Architecture is a common thing and appears to have properties of both Harvard and Von Neumann architectures. It's probably worth discussing a specific device, OS, and programming language rather than having a vague discussion where both parties are constantly describing different goalposts.

It's not that hard to imagine that in a specific embedded device the kernel / compiler have access to write to the instruction memory space while user space programs don't have any API / permissions to do that.

My understanding is that all hard-Harvards are special-purpose ROM-driven systems with static payloads developed on von Neumann computers.

This sort of thing is much more common in microcontrollers, where the code is cross-compiled on another computer and written to flash memory through a debug interface.

In practice, it's usually possible to write to the embedded flash memory via software, but it's much more difficult and disruptive than a simple CPU memory access.

A compiler writes machine code do a file. The OS then loads the code from the file into executable memory before it runs the program. The compiler isn't actually trying to execute the machine code it produces. This is why a JIT wouldn't work - the JIT bypasses the interpreter and produces machine code _that executes directly on the CPU_ inside of the same app.

I don't know any details about Python's architecture, but it's totally possible to write an interpreter for machine code; that's basically what an emulator is. The trade off is that pure interpreters tend to be very slow.