two exceptions to tidal locking
A planet with a large moon could be tidally-locked to the moon.
A planet could have a spin-orbit resonance other than 1:1, like Mercury's 3:2 resonance.
I believe the "habitable ring" concept applies to a planet where the atmosphere does freeze out on the dark side - libation in orbit (like our Moon) could result in seasonal exposures. So even these types of planets could have habitable regions.
Finally, M-star flares get less violent after the first few billion years.