Hi @sacha. The big questions always seem to come down to 1) how obsessed are you with making photos or videos, 2) what's the least in terms of quality can you tolerate, and 3) how much money do you want to spend? I'm not familiar with the D750 or any of Nikon's DSLRs (I last used an FM, FM2, F3, F4). Many DSLRs were shit for live view on the monitor, so using the optical viewfinder was necessary. The newer mirrorless cameras have good monitors, and you can compose and shoot with these, but they are still very small.
The Olympus system is very good. You can make a very compact system with that, and I regret the day I sold my Olympus OM-D EM-1 mark II. The lenses are very small, and the system is quite portable.
If you're not a macro shooter you can get some magnification but not 1:1 with wet lenses like the Nauticam CMC-1 or 2. The 2 doesn't give as much magnification but is easier to shoot because it allows more working distance from your small subject. Nauticam is very good in my opinion, but there are other lenses you can look at such as Inon, AOI, Kraken. I've used the Inon 165, and stacked them, which works okay. If you love macro the Olympus 60mm is very small and very portable.
I don't have any personal experience with the WeeFine WFL01. It should work with the 14-42mm. I use a Nauticam WWL-1 and it works with several different lenses, including the Olympus 14-42mm. The WWL-1 and other wet lenses can be pretty heavy, yes, and require flotation offsets. The best part of taking wet lenses, though, is that your traveling kit may be less bulky. Dense like lead, but less bulky.
I used Ikelite for several years, and found their strobes to be pretty nice. A good amount of power with warm soft light. I did have a DS-51, but didn't like it. Strangely, it was very loud underwater. There are smaller and better strobes available now, including Backscatter's MF-2 (which can shoot TTL with Olympus cameras), Inon and Sea & Sea. The Ikelites will most likely used wired cords, but I think many photographers prefer fiber optic cables, which are durable and long-lasting despite their fragile appearance.