Choosing the right interfacing for a project has always been hard on me, even when I did a lot of garment construction. I have to throw myself on the mercy of the fabric store staff, which sometimes works better than others.
I don't know of anything really you can do to soften it except to use it and love it and wash it. I'd do a practice piece or two not on the quilt to see how your thread and needle combination would work. You might need to use a huge needle (like a 16) and a rather large stitch. I'd try a 12 or 14 needle first and see how it goes, but I would do minimal quilting. You can do grids or gentle arcs, and depending on your skills you may be able to maneuver it around pretty well to do a fairly wide meander.
For designs, you might do some searches on "large stitch" or "sashiko" designs to give you ideas other than grids.
And who knows, there are some advantages to working on stiff as a board things and you might even find it not so bad as you feared -- you might!