Ours is a rock. They delivered 3 before we found the one we wanted. My husband finally went to the "rock" store and picked it out himself. They used a backhoe to set it into place. They put it in before the coping so I'm not sure how labor intensive it would be to do it later. The kids LOVE it! Best of luck finding a solution.

Are the posts embedded in concrete poured around the posts in larger holes? As you can see, there are large gaps between the post and the concrete---which have caused the rot by allowing standing water in the gaps. To replace the posts, you will need to build temporary supports too allow post removal. Best option would be to pour new concrete piers, replace the rotted posts with new ones affixed to the piers with post standoff brackets. Those brackets are bolted to the concrete(usually a center bolt in an anchor) and the post sits in the top about an inch or so above the concrete, with screws holding the post in the bracket. A second option would be to remove the rotted posts, fill the holes with concrete and install a metal plate over the concrete---since the filled holes will not be strong enough to support a new post and the load. Then use the post standoff brackets to install new posts. A third option would be to dig/pour new piers at different locations under the deck and install new posts in those locations, again using the standoff brackets.

Read the installation instruction for the PVC deck you decide to work with. From my experience PVC decking feel better on 12" center framing. We always tear out the frames and start with a new, flat, and properly constructed frame. Old frames take significant amounts of work and time to prepare for a quality installation. The frame is a critical aspect of having a good looking end product. Do it right, it will last a lifetime.

You could combine different orientations. Have a few boards (3,5,more?) running grouch the centre of the deck perpendicular to the house, and then have the right and left side filled with boards laid on a 45 degree angle. You have to play with some scaled drawings to see what would be visually pleasing, and what would eliminate the need for butt joints in the longest runs.