I've been working on the rockstar dolls personalized to resemble my friends (for Xmas gifts). I only knit in the evening and I've managed to crank out a doll a week, knitting about 2 hours a day. And that's with the trial and error of tweaking the pattern so that they look like my friends.
As for mattress stitching, I've been knitting for a couple of years and have always been horrible at it, but since the dolls do require a fair bit of seaming, they've actually helped me beef up my chops. I too learned how to do it from SNB, so go back to Debbie's instructions and try again. My one word of advice is this: don't try to mattress at the very edge of your fabric. Go in to the second stitch and use the ladder (as explained in SNB) as the place that you stick your needle under. It's easier to find that ladder if you go a stitch in, and once you start getting it right, when you give the yarn a tug to tighten up the stitches, that seam should dissapear.
Good luck. You can check out my progress on my dolls at http://www.elizaboothy.blogspot.com