We had this issue pop up twice in our game, ( I'm the GM) due to scheduling conflicts. We collectively ret-conned both characters, having them make their own phase one and two, adding them into phases 3 and four, which were both story arcs shared by the whole group. We are currently in game, playing out phase 5 ( I'm hoping for a more gelled, cohesive group, and it's working pretty well so far.) everyone has a temporary 5th aspect, and I'll be giving free aspect change at the end of the campaign, to reflect what folks feel their 5th aspect should be. (In addition to the end of campaign major milestone.)
It's my first time GMing, and I haven't read on here of anyone playing out the 5th phase; but we wanted to get right into the meat & potatoes of the game. Next session tomorrow night, hopefully everyone will get plenty of compels this time