Ooh yes, Somersaults, star stitch would be an arse to tink back - you can do it, but it would be so slow as to not be worth it for a square, I think. When you get to one of the purl three together stitches, you need to slip the star stitch stitches off the needle, pull on the yarn gently and eventually the three original stitches will appear. But as I say, you would need to do this so slowly to avoid pulling it down too far that for the sake of a square I would just rip it out.
Minnie 
blimey I think this pattern should have come with a health warning on. The obvious mistake in Row 4 is that there aren't enough border stitches
Row 4 ? Sl1, K1, (P5, K2, P5, K2) three times, K2
All the other wrong side (even numbered) rows start Sl1, K3.
However, you can also see the repeat doesn't add up to 13 but rather 14. This rang a warning bell and I can see that in fact, the pattern does grow a few stitches over the next few rows and then loses them again in rows 9 and 11, where you're doing 2 lots of k2togs without any compensating yarn overs. There's no way this won't be wiggly at the edges if there are rows with more stitches on than other rows! This makes me think the whole border bit needs a rethink to compensate.
Looking it again, therefore, I think the repeat is actually 12 stitches before you start row 1 one of the pattern, as you do one k2tog on row 1 but two YOs. Which would make 312=36 + 6 border = 42. And in fact row 24 (the last of the pattern has the stitch count back to 42 as well. Gargh*
So I think the original stitch count of 42 was right, but there is still definitely a mistake in row 4. To reduce the size to 6" you need 212+6=30 stitches, which suggests using 4mm, but the pattern is going to increase to 216+6=38 at its widest point (row 8).