(a) intentional operation requires that the Type Certificate risk levels be maintained ie the MMEL addresses a mini recertification if you like. Hence the various restrictions in the MMEL
(b) FL250 matches older style aircraft limitations in the event of the remaining single pack failing in flight
(c) in the event of an abnormal/emergency in flight, ie not tied up with MMEL considerations, the OEM prescribes appropriate actions to recover the situation but the increased risk levels are part of the gameplan.
(d) it is, however, still a good idea to review the MEL after the QRH items are completed to facilitate consideration of those matters which pertain to certification as the subsequent flight management decision processes may well be influenced by those considerations.