Given the BIG picture of the final product, I'll bet those funny corners on the octagonal deck have kinda melted away as an issue. It sure looks good, man.
I don't see any advantage to add railings. It would be the equivalent of putting railings on a dock. Unless you have a specific safety need in mind, leave it be.
A cupola will lend virtually no function for your gazebo; they are all form. For this reason, if you decide to add it, it must be done in grandiose form, but within scale. In other words, it has to be the big enough to add a grand cap to the gazebo, but within scale. Don't chintz. Also, remember that it will add significant weight at a very high point on the structure. Gazebos and similar post-supported pavillions, are often under-engineered for lateral wind force. Be careful. Those diagonal corner braces might need to be beefed up and structurally attached to add some wind-shear resistance.
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(edit) nix the railings and consider a couple of nice, small benches. If you can engineer the bench supports to allow the seat and backrest to hang off the edge of the dock (taking up no space on the actual deck)....
