With Clint Mansell's contribution being announced in February 2011, and the other composers not starting work until October 2011, it really looks to me like the change to Clint was entirely Bioware's choice, and then the other composers picked up the slack last-minute when he fell through.
I'm not sure if we can read too much into that, the scores to ME1 and ME2 were both the work of multiple composers, it's more than possible that Bioware always intended to get multiple composers involved in ME3 and that Clint Mansell was only hired for the main theme. Bioware got what they wanted out of the deal, Clint's name in big letters on the cover of the soundtrack, while most of the work was done by composers that worked for less money. It's win-win for them.
One thing I really appreciated about the extended cut soundtrack is that it ended with a reprise of the main theme, just like the endings of ME1 and ME2. An End Once And For All was a beautiful piece of music (except the final few seconds of the original version), but it only served as a callback to Leaving Earth. The new final track
works better as the ending-point for the whole series.