I practiced much on the backstitch. To me, it works much better on fabric than knitting seams. However, it is a good seam and I can see why you use it. It was very difficult for me working the seam with right sides are facing.
I think I unknowingly omitted mentioning a possible crucial part of this pattern. Within the last four rows, two of the rows have double decreases at both ends of the rows right before the first and last stitch. Like this:
Row Beginning: K, double decrease, middle sts, double decrease, K
So as I continued to ponder a solution, it dawned on me that such a large slant at the edges might require more yarn strands in the mattress seam to make it stronger in order to hold the seam together. On the last 4 rows, I added strands 4 more times than normal.
I tested out my theory tonight using mattress stitch. Although it is much more difficult to pull the seam together, the stitches now snuggle up to each other as they should. I've only finished one of the raglan seams; I shall attempt this method with the remaining three and hope for the best.
However I think this will work. I can't believe it, because I've struggled over this so long, but I'm thrilled. Also I wanted to share this information with everyone, because I've never seen it discussed on the internet and believe me I searched.
So thanks so much for your help. I can do the backstitch now. It made me think some more and find out why the seam wasn't working.
I hope this all makes sense; if not let me know and I'll try to explain it better.