I went back and looked at the pictures the OP posted and the ridges look to have flat tops to me. I suspect they are fine. The spline would be expensive to replace and replacement may not solve the problem. The OP needs to determine root cause of the problem before he can fix it.
Here's my list of what I think are likely suspects:
1) In he 2nd or 3rd post describing the problem, he stated that he has never used the rear PTO on this tractor, so I wonder if he has released the locking feature on the implement's spline before trying to engage it with the tractor's spline. The first time I tried to hook up a PTO shaft it took me many failed attempts before I figured out how it worked. Every new PTO implement that I aquired had a different style lock, requiring that I relearn the procedure. Our
b5100, little brother to the
b7100, has a PTO guard that is very tight around the stub spline and I have great difficult getting my big mitts in there to attach & remove the PTO shaft, especially on our Secma tiller, which has a ring you must pull towards you at the same time you push the shaft away from you to engage the teeth.
2) I often can't engage my tiller PTO shaft (both the Sicma and the KKII) on either the
B5100 or the
L3240 if the tiller is resting on the tines. This is because I can't rotate the implement spline enough to engage the teeth. To do this I have to lift the implement on the 3pt hitch, then I can spin the tiller tines enough to engage the spline teeth. I have the same issue, to a lesser extent, with our brush hog. Hvae to spine the blades to engage the spline.
As long as the OP is focussed on the "defective" tractor spline, he will not examine the implement spline & locking mechanism with enough attention to discover if that is the problem. He may not be aware how closely the splines must be angularly alligned before they will slide together.
I see this all the time in failure investigations - someone gets fixated on one possible cause and misses some of the clues from the actual problem, prolonging the investigation. I have been guilty of falling into this same thought trap, so it's not meant as a slap, just something to consider.