From my experience, a 9th villager can set up their plot at any day in the course of seven days after the 8th villager has set up their house. If you plan to TT anyway and you want to plot reset for the 9th villager, I recommend the following:
1. Play with your mayor the day the 8th villager has set up their house. Try to talk to all your villagers at least once. Save.
2. Set the date on your 3DS forward 8 days.
3. Create a new player character and check the town for a new plot. It is guaranteed that a plot is somewhere in your town at this point.
4. If you like the villager, save with the new character; if you don't, repeat from step 3.
5. After you saved with the new character, set your 3DS date back 8 days to the actual date.
6. Log in with your mayor and greet the new 9th villager who has just set up their house. Also, someone WILL ask+ping to move out on that day.
From my experience, the "haven't talked in a while" sequence is not triggered at step 6 because you didn't "skip" time with your mayor, only with the new character, and that is only counted as if 1 day had passed (2 days since you then TTed back) concerning the villager conversations - or at least that's what I think. I have done this process for about 30 times and it never failed. You won't have to deal with the annoying "no plot appearing" scenario, and someone is bound to be wanting to move out the first day the 9th villager is there. Good luck
- - - Post Merge - - -
I forgot to say:
When a 9th villager moves out and you want to repeat the above process for the next 9th villager, you have to log in with your mayor 1 day after the 9th moves out, otherwise it's possible no plot will appear at step 3. That's what I experienced. My explanation is that the game needs to "get" that there are really only 8 villagers. If you play with your mayor and there are still 9 (although one is in boxes) and then you TT forward with your 3DS and create a new character, the game still "thinks" there were 9 villagers (although the one who was in boxes is obviously gone) and noone new can move in randomly.
Sorry if that was confusing. :/