Where precisely can Emacs take control of execution to deliver async process output to your filter-function?

emacs_async_yield_points.el

(defvar-local still-waiting nil
  "Whether we are still waiting for a chunk of process output to complete.")

(defun my-worker-1 (process)
  "Do some work and yield to PROCESS output."
  (do-some-sensitive-calculation)       
  (sit-for 1)                             ; A) output can interrupt me here
  (another-sensitive-buffer-manipulation) ; B) what about here?
  (my-worker-2 process))

(defun my-worker-2 (process)
  "Do some work and communicate with PROCESS."
  (heavy-calc1)         ; C) can these simple calc-only calls themselves be interrupted?
  (heavy-calc2)         ; D) how about *between* the two calls?  
  (process-send-string proc "I'm done!") ; E) presumably right here can be
  (my-worker-3)
  (while still-waiting
    (accept-process-output process 0.1 nil 1) ; F) obviously here
    (sit-for 0.02)))                          ; G) and here

(defun my-worker-3 ()
  "Do more work, entirely unrelated to the process."
  (do-something-entirely-unrelated-to-the-process)) ; H) but can this function or its internal calls be interrupted?

And that various emacs internals effectively call "await process_output()" without regard for whether you are prepared to accept it at that moment.

That's not what they do. They start a new local event loop, they are blocking. But yes, if one ensures that no such blocking functions are called, everything should be predictable.