I was answering your specific question about "в or б; и or ы":
Really there is no additional rule for helping where в or б; и or ы must be used apart from this last case for г. х. ж ?
That is why I explained some spelling rules of и/ы (after ш, ж, and after ц), and then added: "In all other cases, it is just phonetic." (it was written referring to и and ы).
There was nothing about unstressed vowels in your original question.
BTW, I did not understand what you meant by "apart from this last case for г. х. ж"? I.e., I did not get what was the issue with г, х, ж?