Thanks.
I understand your concern. So long as a backup is made, I personally feel safe. I usually don't have any problems and am willing to take the risk. I'm an experienced technician, so I know how to go into Safe Mode or even use utilities like BartPE or Reatogo on the rare occasion something gets screwed up. And so long as automated registry scans remain separate from cleanup scans, it would be a purely optional feature.
I understand your concern here as well. However, so long as it's kept an optional feature (e.g. a check box that says "Clean all users?", or even better, a separate check box for each user), I don't see a problem since it would make life easier if multiple users want automated cleanup.
I think that so long as these features are kept optional, CCleaner would remain customizable for each user's needs.