I often use washable Crayola markers. Or you could use water soluble stabilizer. I haven't done this personally, but know that you mark your pattern on the stabilizer, quilt it, then spray lightly on the stitches to remove.
Thank you, Mare Mare for selecting my quilt to be featured! What an honor!
I'd like to put in a plug for the Modern Quilt Guild, which is growing by leaps and bonds. I have made so many wonderful friends and been so inspired by their creativity. You can find a chapter near you at themodernquiltguild.com. There is a new newsletter, and the just announced QuiltCon, next February in Austin, Texas. I first saw the MQG mentioned here on Craftster and I'm so glad I did.
I reduced the pattern when I made one. The only problem I had was the center circles weren't accurate. I ended up appliquéing the ring onto a solid background. Could have been my math. Could ave been my cutting. The wheel went together just fine.
I think American buttercream is always grainy tasting. I don't think it's you. It's more work, but you could try Swiss or Italian buttercream. They are sooo much better. My favorite is the mousseline buttercream from the Cake Bible.