Oh, and because I didn't address it directly in the longer answer...

> Does the interpreter hit the WHILE token, goes into IMMEDIATE mode, remembers the location of the WHILE, then dispatches all the subsequent code, until it hits the REPEAT token, then branches back to the WHILE?

Yes. The beauty is that, in the context of a threaded code compiler (which, again, I encourage you to use as your default model for Forth, even though other options are possible), WHILE just pushes the address of the output code stream onto the compile-time stack. REPEAT expects the address to be on the compile-time stack and compiles a conditional jump to that address. This obviously and trivially provides support for nested control structures of all types; as long as the word that pushes compile-time data onto the stack is correctly paired with the word the pops it, we have stack semantics for nested control, which is exactly the expectation. So while your description is completely correct, "remembers" is almost trivial here -- "puts data on stack" is the primitive operation of remembering anything in Forth, and that's all that's needed here, no fancy data structures or look-aside buffers or anything. (Note that the compiler does require at least two non-stack data structures, the symbol table and the output code stream, but those reflect real domain complexity.)