Why does this hollowtech crankset have play?
The marks on the splines look about normal. It's hard to tell absolutely from the pics. You're looking for any sign of deformation or rounding, which occurs if the pinch bolts are loose while riding. Always make sure the pinch bolts are nice and tight, 14Nm or about as much as you would ever put through a long-handled 5mm L-wrench.

You need to be able to torque-check your BB cups to go further. If one is loose that can cause all the issues you're experiencing. Replacing the BB entirely at this mileage is within reason too, although if it still works and feels great then by all means keep with it. Note they can have issues with looseness that's not accompanied by poor bearing feel. For example, the black plastic "top hat" of an HT2 BB can get cracked or come apart and preload will be erratic like you're experiencing even if the bearing feel is good.