There's threads on using an O-ring for over under on this forum as I recall.
The fisheye converter is a different beast to the 16-35 rectilinear as it provides a 180° diagonal field of view vs the max field of about 105° for the 16mm end So you have to be very much closer to your subject and much more concern about what intrudes into the edge of the frame including you own feet if you are taking pics on land.
Regarding the advantages of the adapted 28mm the only advantage is cost, the lack of a native fisheye is regarded as a weakness of the Sony lens range for UW work, whether that's a problem for you will depend on what you are shooting (how big it is) and whether of not your subject likes having a dome port placed right in its face. If you look at lens reviews you will see for example the Canon 8-15mm fisheye gets better reviews than the adapted Sony offer, with the edges in particular being much better.
I see that you say you settled on Sony as "the best " camera, presumably you mean the best image quality. Actually getting that advantage when you place the camera/lens behind a dome is more difficult than you might think. If you look at the premise of the WACP it says that the image quality behind domes is degraded to such a degree that an old 28-70 f3.5-5.6 Nikon kit lens behind the WACP outperforms the latest 16-35mm f2.8 zooms because the optics with the dome degrade the image to such an extent that the old 28-70 produces a better image. The logical leap from this that that extra last bit of resolution is not obtainable behind a dome underwater. I would advocate looking at the UW system choice from a total system perspective and that includes UW-suitable lens availability, port size and availability, Housing options and camera body, taking into account your subject matter rather than what can I do with the best camera body available. Bottom up rather than top down.
I shoot Canon on land and decided m43 Olympus was the best choice for me, housings and lenses are significantly cheaper, dome ports smaller, lens line up complete all a lot lighter and easier to handle underwater. Not saying that's the right choice for you but would encourage you to consider from a total system basis.
A few more questions about your shooting, you say you are shooting in streams, assume you are in the water shooting available light (no Strobes), a lot of over under. How close can you get to your subject and how big is it? The over-unders may present some difficulty because the UW portion is a virtual image very close to the dome and the above water portion may be effectively at infinity so you need to stop well down to get both in focus. Bigger domes will help with this as will shorter focal length fisheye lenses because of their inherent DOF. The UW portion will be darker and balancing between the two may be difficult without strobes depending on the lighting available.