Does the kit come with the screws and washers to fix the top or are they purchased separately?

Kit Includes all the hardware to properly fasten the Resin Dock Top to the frame.

How thick is the surface?

1/8" thick at surface with a reinforced web of 1" square cross pattern underneath.   Will Not deflect, crack, break or fade.

Can this be installed over the top of an existing treated wood deck?

We do not have any experience installing the Resin Dock Top over an existing wood deck. If you attempt you will want to make certain the wood deck is level, secured and pay close attention to where the holes to fasten the Resin Top line up with any screws in the existing wood deck. Also you may have to use longer screws than what are provided in the Resin Dock Top in an effort to completely fasten the Resin Top to your Dock frame and not just the wood decking.

Can you attach chairs to this dock? We want to use this dock for fishing. Can you bolt standard dock chairs into the dock top?

We are not familiar with how the chairs would attach to your dock.