I have the Sony 10-18 f4 and I really like it. I have not shot it underwater yet, but I have a trip coming up next month on a LOB, so it will get a pretty good workout then. I don't plan on getting the zoom ring for it, I will just set it at 10mm and shoot. If I need to crop in Lightroom after the trip then I will. One advantage that it has over the other two lenses you list is that it is fully compatible with the 7 inch dome that you already have.
As for the 16mm, I have not used one, but I have not heard many good things about it other than its size. It seems to be very hit or miss as to how good the focus is with most of them being pretty soft, especially along the edges and in the corners.
I love Zeiss optics. If cost is no factor and you have the room to pack another dome port and extension, then I don't think that you could go wrong with this lens. Keep in mind though that it will not fit in your 7 inch dome (it is too wide) and you will need the N85-N120 adapter and then the 180mm Dome port. This will add significantly to both the cost of your set up and the size of what you need to pack.
Very few things in life are "perfect". I think, however that given the options that you list, I would go with the Sony 10-18mm f4. I really like it above the surface, it has the Sony OSS system, and it uses the same dome port as your 16-70mm lens. It may not be perfect, but I think it would easily be your "98% solution".
