I have a different take on the M2/M-Inf. connection.
M2 occured, and at the end Durandal goes off with the Marine, and the events of Infinity unfold. However, after the Jjaro space station situation in Infinity, but before the end screen -- this is where scenario's like my own (WMaiD), Rubicon, and Marathon:Red take place.
At the end of time, after all the adventurs are said and done - we're seeing Marathon:Infinities closing chapter screen. Durandal and the player find the escape hatch from a dying universe, and become 'gods'.