The game days run between 6 AM and 6 PM, as you probably already know. It sounds like you TTed back the first time, since presumably you got Muffy to move in on the 19th after 6 AM. When you go back in time like that, the day counts it as one day forward (for example, if you TT back two days or two hundred, the game counts it as one day forward). This means Muffy ended up plotting on the previous day, and so when the game advanced a day, her house was already set up.
Maybe I'm just misunderstanding what you said... when you saw her plot in-game, what hour/day was it?
I don't plot reset but the last time a villager moved in I wanted to make sure she wouldn't move in on top of my orchard, so the way I did was just TTing one day ahead (the day she'd be plotting) using the 3DS clock and starting with a new villager. I didn't bother going to 5-something AM since I don't see why that step is necessary and it sounds like it'd just make it more likely to mess things up. Then when I saw her plot in a good spot, I saved, re-adjusted the 3DS clock, deleted the new character and loaded the game as my mayor.