A: 2" for standard rung.
A: 12" between rungs
A: You would want to use 1/2" concrete anchors to attach this to a concrete wall.
A: The ladder requires four bolts which you provide depending on your dock. These bolts go through the horizontal bars that lay on top of the dock or in your case, the seawall. No bolts are required on the vertical bars that support the steps. The steps will be flat against the seawall, so you will only be able to fit the front of your foot on the step. I would look at another type of ladder with deeper steps for your situation.
A: You need 4 bolts and 4 nuts. The length will depend on the board depth of your dock surface plus the depth of the horizontal bars. We added boards under the dock for extra support. No bolts are required for the vertical bars that support the steps.
A: The mounting hardware required depends on what material and type of decking it is attaching to.