When the first two seams are nested, the cross seam should be pressed in opposite directions, so that all the seams rotate around the block. Your iron can go around and around in the same direction. That way when you sew to another block, all those seams will nest.
That center "pop" is easy to do. I grasp the seam with my thumb nails and pointer finger and twist. Most times and seam just "pops" open. Sometimes I can take a pin and undo one more stitch. But do not cut those threads. They need to stay there to secure the next stitch in that seam you just popped.
My stack 'n' whack was made that way, and it was very easy to sew one block to another. All the seams nested. Just be sure you go the same way on all your blocks; either clockwise or counterclockwise. I see in the pictures of OP that each one is different. The black and white goes one way and the yellow goes the other way. Works for her.
Last edited by maviskw; 06-04-2016 at 04:35 AM.