1) You saved the map from a non-finite resources game. The value of "Earth was then set to 120/90/120 or something.
In finite resources games, those numbers become 1.2K/900/1.2K or something.
Create the map for a finite resources game, then save, and when adding/editing planets, you'll have to use HUGE value numbers.
2) You must delete all the objects inside the system first. Also delete both sides of warppoints leading in and out of the system. Then it should be OK.
3) Not that I know of, but there probably is at least some threads on it. Try the search function (near the post reply/thread buttons)
4) For rings, yeah. What you need to do is edit the PlanetTypes.txt, and make a large to huge world that uses the right picture.
5) TRY: Add the Moon first, then the Earth.
The moon should be small, and earth medium. (Medium can be a homeworld size)
6) Abilities.txt in the data folder will tell you about the abilities, and what the values mean. The description is totally up to you.
Be aware that not all abilities will work on all types of object, even if it would make sense, like "planet-shield generation" on a planet
7) Yep. That would be a data mod.
Do you know how to use the Path.txt to keep your original game and mod(s) separate?