METAR can be automatically generated by a station, generally every hours but METAR can also be created or amended by a weather specialist. It’s not uncommon to have METAR updated at half-past (xxH30) if needed, due to weather change (no necessarily “severe”) and because someone is capable to update it. Sometime a METAR can also give a short forecast.
Yes and No. An ATIS is made of data weather information but not only. So an ATIS can be updated independantly from a METAR.
By the way, your original question was linked to the ATIS Letter. You can image a scenario were two station providing an ATIS don’t start at the same time, not too mention that one can be 24/7 and the other day only.