How could they build so fast update/ repost.

I think that's highly unlikely.  I'm inclined to think more like the MB planned to come but got left behind in the Milky Way.  As dmc1001 suggested, an assassin was sent out on the Nexus with orders to murder Garson on arrival (perhaps so that the MB could take over as he/she arrived on an ark - which were always planned to arrive later than Nexus).  What went to shit for the MB's plan is that the MB didn't manage to get onto that ark.  MB may have then made a second plan to get on board the Quarian Ark... unknown whether or not they managed it.  If it's Henry Lawson (who's stated to be the richest man in the galaxy by Niket in ME2), then he obviously never makes it to Andromeda despite whatever plans he might have made to get there.

My primary theory is that MB is a volus... or rather a consortium of volus.   The volus are the species that control the banking and investment system in the Milky Way, so it seems likely that they are all secretly wealthier than the OT reveals.  It also makes sense that they would be planning to be on the quarian ark.  We also have a statement in ME3 from Barla Von that he's planning to leave, but then winds up being on the Citadel a little longer.  We have statements from Rupe Elkoss in ME3 that he's made is own plans for surviving the war.  If Bioware wants to add some of "The Godfather" intrigue into ME:A's story, then I think the Volus family are the perfect MB candidates... Barla Von, Rupe Elkoss, Pitne For, and Din Korlack.

That makes more sense...just hadn't heard that particular reference outside the MB.

The quarian ark was delayed, its possible he left with them.  Its possible he has a separate ship.  Its possible he faked his death like you killed a clone in me3 or some other double.  I find it highly unlikely he'd be the benefactor but you can write you way around almost anything.