Britney Spears' ex-husband -- not Kevin Federline, the other one -- is back in her life and seems to be shooting his shot for another chance ... he's joining the #FreeBritney movement. Jason Allen Alexander -- who you may recall was married to Britney for a whopping 55 hours after their wild, 2004 Vegas wedding -- was outside the L.A. courthouse Wednesday where a hearing took place over her conservatorship ... which has been in force since 2008 after her breakdown.