I have changed my approach a bit on this. Rather than poke away at the standard scenario, I took one of my mods to it and generated a very large archipelago map, and then added a few additional resources to make things a bit easier as New Zealand. This way, I can learn some of the quirks of the scenario without having to worry about being immediately attacked.
1. Forget the coral reefs making the Sea impassable. The Sea was set to be impassable, and the only Sea on the original map was a small area between Australia and New Guinea. That took a while to figure that out. However, that does mean on my archipelago map, the AI really cannot bother me for a while.
2. I am auto-generating Great Military Heroes from the Supreme HQ, and that has proved interesting. I do need to check this on the original map to see if it works the same way. I have them set to 2 movement with all terrains as roads. Without being an army, they have only 2 tile movement, but go everywhere except obviously Water tiles. They do make excellent scouts, and I have popped a lot of goody huts with Tech Advances, so I am up to 1943 already in terms of Tech. However, when I accidentally added a Matilda to one of them and made it an Army, it now has a movement rate of 6 tiles, treating all Terrain as roads. Quite strange, but very useful. I will add some more Matilda Tanks to my one unit army and have some fun.
3. I am changing the combat values as I need to, a few at a time. Eventually, I will get them changed, but I may simplify things by simply ignoring some units.
4. Keeping the citizens happy is proving to be a major problem, and the building chain of buildings to make citizens happy is proving to be a real headache, at least in the standard game for New Zealand. It may go better with nations with more cities to generate revenue. I am not sure about that.
I will keep plugging away at my custom map, and get more information. As I said, it is easier for me to do it this way and find the quirks than working on the given Pacific map. Viewing this as an Alpha version, also helps.
Edit Note: One odd thing about the map that I generated. Even though there are 7 civilizations in the game, there are only 3 sources of iron, and the same holds true for Oil, Aluminum, Coal, and Rubber. This is going to make building units for some of the AI very difficult. Also, even civilization has a resource keyed to it, and those are limited as well. For some very odd reason, as New Zealand, I got the starting point with all 5 Japan resources, leaving Japan in a massive world of hurt. Needless to say, I think that the game has a quirky sense of humor.