That's really some great and thorough work. I can only hope that this thread has gained the attention of the core folks. I know that several people (including core folk) have complained in these forums about having to address deleting permissions explicitly. I think it's great that you took the time to thoroughly analyze *all* of the shortcomings of group deletion.
Many of the people that would know if your fix is correct are very busy at the moment preparing the 5.0 code for release. It would be a shame for this to fall through a crack because it's not addressed at this point in time.