1) Yes, but only on 3.60 or any other enso compatible firmware (1.50 - 3.65)
2) Yes
3) Updater reflashes the MBR - enso is removed/deleted, so no, you cant update. And you can use sd2vita on 1.69 - 3.65 with enso, and on 3.67+ with the new hack (but keep in mind that you need a original sony mc...