Ok, an update on this.
I bought the mount base and the 165 fisheye lense. I was originally going with the 105WA, but changed my mind and got the fisheye. Haven't tried it underwater yet.
I have the sea&sea light tray and arm (generously donated from my brother-in-law as he upgraded to DSLR). That tray has a thumbscrew in a slot. The thumbscrew was not long enough with the base installed. No problem, I drilled out one end of the slot to release the screw. I had a few different thumbscrews of varying length to try and got one to work, although it required some filing of the plastic collar to fit the slot.
The sea&sea has kind of a rubbery base which holds the camera fairly well.
However, the mount base has extra holes in it which you can put a bolt in. I may drill another hole in the tray to allow me to put a bolt though. This would ensure no rotational movement of the camera housing.
I haven't seen any other filters other than the 10bar AD red filter. I'm leaning towards auto magic filter for use in video mode. That would allow use with the FE lense. I wonder how that would work. Of course, the downside is you're stuck in video or available light stills for the whole dive.
I don't know of any camera that allows manual WB in video mode.
To answer another thing someone mentioned, I'm using a strobe (maybe 2) so I need the tray.