"I'm wondering is there a way to TT back to the last played date and pretend nothing happened while I was gone?"
There is c: Set your 3DS clock (the one in your System Settings) back to your last played date, then load up the game with a new save file. It won't be as if nothing happened - meaning that there'll still be some random flowers or weeds spawned, but if you do this you can look around on the save file and make sure no one moved out. If no one's gone, than plot your house and save and quit. You can delete the character later, but the state of your town will be saved. If someone is gone, just quit the game without saving and redo this until a character you want isn't gone.
After this, load up your mayor, and when Isabelle gives you the date, switch it so that it matches the one on your 3DS clock. This way the state of your town that it was in when you saved on the new save file will still be there when you load up on your mayor.
Since you're probably gonna wanna TT back to your own time at some point without risking losing villagers, you can do this method. Basically, just TT ahead by one day again and again until a villager pings you saying that they want to move. Make sure before you TT to another day that you've talked to everyone, confirmed rumors (if you're getting the generic "Rumors are scary, aren't they?" dialogue than no one is moving), walking in front of everyone, and have saved and quit a few times.
After a villager pings you saying they want to move, you can either confirm or deny their request, doesn't matter either way. After that, save and quit, and then TT back to the correct date. The game's set up so that no villagers will move out if you time travel ahead on the same day that one pinged you to move out.
"is it likely that only one villager has moved out in this time? or am I gonna be in for a game-breaking wakeup call?"
I can't say if it's likely or unlikely, since villagers can move out at such random rates. Worst case scenario would be a villager decided to move the day after that you stopped playing, and moved out on the other 5 days that you weren't playing. Again, that's worst case though.
OH, and only ONE villager can move out a time when you take a break. You could stop playing the game for two full years and I'm pretty sure you'd only lose a max of one villager. When I was messing with my old save, I time traveled all the way to the year 2040 and I only lost one villager. I don't think the game is programmed so that you lose more than one during your gone time.