Taste is personal preference! I use Satin Ice and everyone loves it. Wilton on the other hand...YUCK! I think you just have to try some of the different brands and see if there is a brand that you like.
I would definitely say they used thinned RI in multiple colors and then used a toothpick (or other tool) to drag the icing around. If you search the galleries for fall leaves, you will see several examples of people using this technique! HTH
It doesn't really matter. If you use unsalted butter, just add salt and if you use salted butter leave the salt out. By using unsalted butter you are more easily able to control the amount of salt you add. HTH
It's funny that you should ask this! Just last weekend I ran short of chocolate chips for my chocolate chip cookies. I had some Hershey kisses, so I chopped them up and dropped them in. It worked great! HTH