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 :)
Just wait till one of them confesses and then if the other side me says "sure" then they will become sweet hearts 

Hope this helps