Question: How can I get miis to instantly be sweethearts and to be happy, no matter what the compatibility meter says. I do not want to have to do anything with the time as it has already been played with.

Answer: 
It's all random. If you pick "Go for it" when a Mii wants to confess to another Mii and the other Mii says something like "Sure", then they will be sweethearts and even one of them might want to [propose!](http://tomodachi.wikia.com/wiki/Proposal).

Let me know if this doesn't work for you or this answer wrong

Hope this helps :)
You have to wait until a mii says that he/she has feelings for another Mii. When the other Mii says "Sure!" after the mii confesses to him/her, they will become sweethearts.

Hope this helps!