Although you normally leave the paper on till the very end, I would CAREFULLY remove the paper from the centers. You don't have to remove it all, just an inch or so from the center. Then see if you can fan the points like you have started in the last photo.
A bigger potential issue is that I'm pretty sure that before your final seam you should have 2 perfect rectangles. If you look at the bottom piece in the 3rd photo, you can see that the seam allowance has a big jog in it. I haven't done this specific pattern, but have done similar and this last seam has always been a straight line across the 2 halves. Check your pattern to see, as this one may be different.
And finally, I would take the Carol Doak's "mastering foundation paper piecing" course on Craftsy. Great class!