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: 
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!
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