Good news. Ryan just tested and confirmed that the Nexus multiport for the 70-180 does work properly with the Seatool Nexus port adapter. The most compelling reason for me to use a Nexus system now applies to Seatool too.
All they need to do is a D700 housing for that D700x that's coming and I'll be set. 
The Seatool housing is very small yet it supports a surprising number of the D300 controls. Ergonomics are pretty good but you have to get used to a gripless right hand or else give up the back focus control. Seatool offers a range of port adapters so it's easier to get a wide variety of domes compared to Nexus. I'd suspect that bouyancy issues are similar for both manufacturers. You need to plan on some floaty strobe arms.
I got the Seatool because of the back focus control, the complete support of functions, the popup flash capability (that Nexus also provides), and the support for several manufacturers' port systems including Nexus and Inon whose macro ports are most interesting to me.
1: Probably not a lot different.
2: Nexus ergonomics are good but doesn't offer all controls I want. Seatool does.
3: Similar. Seatool allows control over popup flash. Idk about Nexus.
4: Seatool supports all Nexus plus several of brands as well as its own.
5: Same as 4.
Seatool would be my choice. Hard to beat a Nexus grip though I've adapted an Inon grip and it's pretty good.