Using the 45 viewfinder for macro does take some time to learn, but makes it much better to frame shots once you get used to it. When I first read that you were bumping up against the viewfinder and that it caused a leak I thought you were talking about the housing - but then realized you were talking about your mask. My mask is up against the 45 viewfinder on every shot - haven't had this problem before...
45 vs. 180 viewfinder:
I own both, and have used both, but after getting used to the 45 I've basically put the 180 on the shelf and use the 45 exclusively for macro and wide angle. Lots of folks are going to tell you that the 45 is for macro - the fact that I can rotate the viewfinder so I can easily shoot either portrait or landscape while basically staying in the same orientation to my subject ends up being great for wide angle. Also - when shooting subjects at the surface (my family was in both Moorea and Tetiaroa last year shooting humpbacks) the 45 is perfect, because you can float on the surface and shoot with the camera just below the surface (although we spent a lot of the time either shooting slightly downwards where the viewfinder doesn't help as much, or shooting level while free diving to about 10-15 feet - where the viewfinder is fine).
Lighting while shooting humpbacks and sharks in French Polynesia:
While shooting the humpbacks I think you are already know that you won't be carrying any lights (strobes, lights, etc.). You are going to swim, and swim some more, and yet swim even more to both get close to the whales and position yourself so that can shoot side on or face on. You are going to want to push the minimum amount of camera gear through the water. It's all ambient light shooting.
For shooting the sharks at Fakarava and Rangiroa - I would absolutely take your strobes. Even a small amount of strobe lighting helps freeze the outer edges of the sharks against the water background. Because you will be shooting through several feet of water it will make a big difference (and if you shoot on the outer edges of the passes, you will be deep). Weight restrictions are reasonable on the inter-island flights, and scuba divers can check in an extra 5kg of baggage (on the larger planes).
I've been to the Maldives a number of times, and never had an issue with having my camera handed down to me (via the braided handle clipped near both ball mounts). Some of the dives can be "negative entry" - but never to the point where you couldn't quickly return to the surface to collect your camera. Personally, I would never jump off a boat with my housing, strobes, etc. I have made some "quick entries" sliding off a boat to get shots of dolphins or a manta swimming by, but that's been while snorkeling.
Last - you can shoot as many or as few subjects as you want on a dive. However, Anilao is very much a u/w photo destination, and the guides are all (very) used to finding a subject for photographers then expecting them to spend 1 minute - 20+ minutes shooting that one subject. When you are shooting they usually wander off to find the next subject for you. U/W photographers usually only have 2 speeds on a dive - slow and stop. If you're diving with a (non-photographer) buddy, this can cause some tension... as you shoot more with your DSLR you will notice that you will start slowing down as you dive - and your buddy is going to end up waiting for you (sometimes a lot, as you find subjects you want to spend time with). My wife has put up with ~20 years of this, and I've noticed that I shoot a little less on a dive now when we dive together (it's a compromise). When I dive with either our son or daughter (who both also shoot underwater) we'll stay shooting the same subject for minutes at a time (and cover less distance).