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: 
Well it will take one day to one week if the miis relation ship is working out just check there friendship rating if they don't have a sweetheart.
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