The 105 Nikon lens has a minimum focus distance of about 1 foot (30 cm or so) measured from the focal plane line on the camera body (not the front of the lens). At that distance you get true 1:1 i.e. a beastie 36 mm or so across fills the frame. What you should do if you are interested is to mount your camera on a tripod and shoot a ruler at various distances from the ruler and you will see that even relatively large movements away (say an inch or 10%) you will still get 38 mm or so filling the frame. If you play that way you will find out how important (to you) that last little bit of magnification is.
I have shot with the canon 60 and adapter. What I do is set the focus long before shooting; and use the manual move the camera forward and back to get the shot. Works fine if a bit slow. Trying to use autofocus with any of these adapters (so far) seems hopeless.
Dome ports allow you to shoot wide angle photographs more easily. You can use them with any lens but in general you want them for wider lenses both rectilinear ones and fisheyes. The dome is acting as a big optical element and it allows you to shoot a wide scene but of a "virtual image" that is created by the dome. The virtual image is between 1 and 15 inches from the dome (depending on the dome size, larger domes lead to virtual images further away).
i can highly recommend the workshops run by Mike and Luca if you can make them, unlike many other workshops they don't take cameras with them on dives, but are there to help the students. Otherwise you can go to a Underwater Photo society if there is one near you, at LAUPS we help individuals all the time with everything from the basics to advanced stuff.
I took my 7D and Oly EM-1 on a trip to Indo. At the end, most guests thought the EM-1 photos are better. As for macro, a lot depends on what you want to shoot and how small you want to shoot. The Canon 100 and the Canon 60 are great lenses but shoot 35 mm full frame. The native OLY 60 shoots 17 mm full frame. Any diopter improves things proportionally, i.e. a 2x diopter for the Canon gets you to where the OLy is natively and the 2x on the oLY gets you to 8 mm full frame. That being said, and if you shoot raw, to me at least you can't tell the IQ apart very much. You could go Canon 5DIIIS with 50 MP and crop like mad, but I would rather carry around the smaller/cheaper system.
P.S. Going to God's Pocket in Sept, hope it is as good as the Alaska Trip.
Hope you are well and diving a lot. For me if you are shooting tiny stuff (like the small spiny lumpsuckers) then the olympus om-d e-m1 (or the new em5-II) is the best option with the only 60 macro lens. Out of the box it is 1:1 (on the smaller sensor) meaning it is 2:1 compared to full frame and 1.5:1 compared to crop sensor DSLR. Then add the CMC (or my old SMC) and you are at 4:L1 compared to full frame. I have been shooting that for a while now and for tiny stuff it is quite nice.
You can get pretty nice results from them if you use the focus stick religiously, the depth of field with them is ridiculously shallow and you need to be at just the right distance. that being said, if you nail the distance the results are quite nice.