I don't know if this would actually help, and I suspect that it will be much harder to do, but how about splitting your five sided patchs by a curve from the pointy corner to the center of the center line in order to make two four sided patches?. That way you don't need to use Fill Surfaces, but can use Boundary Surfaces.
Not a losing battle. I don't like Sketch 14, and I think it is what is causing your issues. No matter how you look at it, something in Boundary-Surface3 is creating the wrinkles. Why did you add Sketch 14? Was it added just to smooth the boundary, or do you want the model to follow that curve specifically?
What I would want to do is instead of creating a boundary surface for just the bottom half, create it for the whole length, and then trim it down afterwards to just be half the height. I think that will fix your corner bunching, by allowing the UV flow to be non-perpendicular to the existing edges.
Also, let me add that you should only model half of this, and then mirror the whole thing at the end. At the transition between the left and right, make sure it has a "tangency" constraint, and on the opposite side add a small helper surface that is normal to the left/right plane.
For a quick fix to that boundary surface, change the "Dir2 curves influence" from "Global" to "Linear".