My two did not have relapses. But that depends on what you consider a relapse. My oldest (5 years) had an accident at daycare a few months ago. It was truly an accident though... she thought it was just gas but unfortunately she ate something that didn't agree so it was far more than gas. Poor thing.
Anyhow, that was once in over two years? Before that she did great however every once in a blue moon she would have an accident where she just got so distracted that she didn't realize that she had to go until it was too late. I don't consider the accidents a relapse. It's just a part of the learning and figuring things out.
For the night-time, I waited for her to go a month without an accident before we switched to panties. She had lots of accidents though during the night. My second LO rarely has accidents at night but I will probably do the same with her as it work out really well. I hate washing sheets =)
For the pooping, my first did not have trouble letting me know when she had to - but we started poop training when she was 9 months old. Her sister hates going poop and would hold it as long as she could. However, as with her sister we started her poop training early so she's never really pooped in a diaper. To get her to go regularly I ask her poop every other night if she doesn't ask herself and then she gets a treat of some sort. No poop accidents though so I don't know if that helps you. I think if you "train" them to go before they have to go, then that might help.
Good luck!