I'm not sure I understand. There's no 'reset' per se really. The PR requirement is a rolling one (two years in most recent five on any day of examination, going back from that day of examination).
-If they come and land to become PRs and stay even a day, that sets the clock rolling. They can go back and come to settle later, but if they stay abroad more than three years total (in any five year period), they're out of compliance with the residency obligation. Worst that happens if they don't stay compliant is they lose the PR status (although not quite as simple as there can be some inconveniences and complications).
I don't know the citizenshp aspect as well but I don't think it's complicated: when they get 1095 days in Canada, again looking back five years from any day, they can apply for citizenship. Until they get citizenship, they need to stay compliant with the residency obligation.