A)Longer, heavier lines will slow it down, BUT you risk giving up control if they get too long. But since you're not doing tricks yet, that's probly not an issue. B)Tails are a cool idea, probly the best in your case, and they look really good and add fun. And I mean long tails, don't be timid here. C)I doubt if you could spin with them, but you can also add "nappies", they look like they'd be easy to make. Check this page out :
http://www.kites.org/kitesandmusic/workshop.htm(the link at the top doesn't work, but you can scroll down.)