What you have is a reflective metallic treatment which I think originated with the SE and obviously extended to the Plus of the same era. I agree with JDW, please post your results to the Retrobright forum as this specific type of coating is as susceptible to the H202's corrosive effects as the metallic badges, which you wisely removed.
It is important to note, that this has not been a problem as confirmed with the thicker gray metallic paint coatings used from 1984–1987 and 1989 and later, which are perfectly safe to immerse for extended periods of time. Whatever the reason for Apple switching to the reflective metallic coating in 1987, they switched back to the thicker type by 1989 as my SE/30 and Classics both have the traditional thick gray paint.
Also, the effect seems to be compounded somewhat by your lack of sunlight, without otherwise delivering the desired de-yellowing. It has been noted that the best results are achieved on bright sunny days, warm or cold. I'm certain that the effects on the metallic coating would have been significantly mitigated if it had not been exposed to the H202 for such an extended period of time, and your Mac would be noticeably "brighter".