I think Stacy and JD make very valid points.
I am going to include a few of your photos with a few lines and arrows added in. Please know that I only want to help you build a great slip joint and I am not pointing out what I see as design flaws as a way to denigrate your efforts but to help you on your way.
Let's assume your heat treat company accurately reported the hardness. If correct I feel you're fighting a bit of an uphill battle with too soft of a spring. Can a slip joint work with those harnesses? Yes, with a proper designed tang and spring with a light enough pre-load.
The first picture I marked up is a tang in relationship to your spring. The arrow points to "high" spot that should not be there. That high spot in that tang is causing all kinds of problems. The only spot of the tang that should ride on the spring is what AG Russel calls the "opening " torque point. The back of the tang should be a straight line from opening to closing torque points, NEVER convex as in your photo. Slightly concave can be gotten away with.
This second photo illustrates the fact that I think your placing the blade tang pivot hole too far off center to the lower extreme. This in effect leaves your back spring too wide which stiffens up the spring considerably. I added lines more inline to where I feel the pivot hold should be placed. Of course there is quite a bit of latitude and depending on the blade shape, frame shape , primary or secondary blade this will change. Point being is that your back spring is very wide and gets really resistant to flexing.
I also feel as if your center pin hole for the backspring should be closer to the edge of the back spine, but to be fair I do know know how closely they are shaped to your frame profile. Remember a back spring flex's primarily from that center pin forward to the tang, but also , to a much smaller degree from the center pin to the rear pin.
The last item I will touch on this evening is your sanding method using the 1-2-3 block, granite and sandpaper. In theory it should keep things square, in practice I have my doubts. Removing sufficient amounts of steel in the 60 plus Rockwell C scale to fit up a tang has got to be a time consuming effort along with keeping the tang tight against the 1-2-3 block preventing any minute introduction of a radius or out of square condition.
Last picture is a great simple resource giving a good idea of what a round tang and square tang should look like and nomenclature.