How do you get married in less than a year and how to do you put in the cheat ccodes?

Hard devotion and giving them their favorite items. Although, some bachelors you can't. Ex: Cliff. His favorites are cooked foods, and as we all know, you don't start out with a kitchen. In this case, give them what you can, as long as it isn't one of their dislikes.
And I don't know about the cheat codes, as I've played the game without any. xP

Where does Kappa stay/live?

Like in FoMT, Kappa lives in the lake where the Lake Mine is. Just throw a Cucumber in. The only time you can't reach Kappa is in Winter, when the lake freezes over.

How do you get children?

Be nice to your husband for about a season or two and you'll feel ill one day. After another two seasons after finding out about the child, she'll be born.

Why won't Rick marry me, we are already at a red heart and have been for 3 years?

You have to have seen all of his Heart Events and have all of the items off the TV Shopping Channel. To do this, you need your house upgraded to its third stage.

http://www.fogu.com/hm5/
is a good site to check out for any more of your questions.