1) 3.65 Enso is somewhat useful, if you want to play 3.65 games reliably. While Maidump can now dump 3.65 games and make them playable on 3.60, you still have all the same issues that every other Maidump game has (like putting the Vita to sleep while playing can delete your save). 3.65 games can also randomly crash and have various other issues while being played on 3.60, whereas they play totally fine on 3.65. If you don't care for 3.65 games, then it's probably not worth it to bother updating to 3.65.
2) You can't reflash the 3.65 exploit, however you can restore your Vita to factory settings as many times as you'd like without it modifying Enso. This is because Enso is now stored in vs0, which doesn't get touched with a factory reset. So long as you don't go messing with system partitions or running homebrew from flaky sources, you should never lose Enso on 3.65.
3) And yes, when you get your SD2Vita you can just copy everything on the original memory card and place it on the SD2Vita and, once mounted to ux0, everything will function as normal.