Well, I finished it......
I used Dawn's method (connect 2 sides of the frame, connect those to the center square, repeat with the other 2 sides, connect the remaining 2 seams), which seemed to make the most sense to me. Everything was straight as I went, but in the end one side went very crooked.
For now, I'm leaving it. I did a lot of taking apart and re-sewing while working on it, to get the points lined up as good as I did (which still isn't perfect) and I'm just done with this block for now.
Since it's for a diary quilt (25 six-inch blocks, each one different) and this is the hardest block in the quilt, I'm just going to set it aside and re-visit it once I'm farther in on the project. I'll work in chronological order and when I reach the point where this block actually fits (it's something like block 18 or so....) I'll pull it out again and try to fix it/start over with it.
But, here's the picture anyway, since you were all so helpful along the way. Luckily, all my other blocks are just squares and triangles that sew up into squares/rectangles. Nice, straight edges and no more Y seams. So, the rest should be easy!
Thanks again for the help!
-Heather, hand-piecing quilting newbie