One thing you can do, is (before you clip or turn) machine stitch just short of the seam/turn line (like a thread's width and with a very small stitch). The easiest way is to trace shape onto your fabric, then stitch, before cutting out your shape. Clip JUST to that stitching line; don't cut the stitches. Then when you turn, you'll just catch that stitching, within the turned seam. The stitching will give you a guide, on which to turn, and will also help keep the seam from fraying too badly. As you sew into the tight inward curve, take tiny stitches.
I've never had a problem with Fray Check yellowing BUT I don't generally use it, where it'll show. It does make the treated area just a tad stiffer.