In regards to #1, I think its you need to have 100k tons of warships in sea area the ports are part of, then what area you are trying to conquer will have a port with a small circle. You must have your fleet within that area.
If you wanted to invade Saipan you'd need 100k tons of ships in a fleet in the Philippines Sea. Then you'd need whatever tonnage close to the port, within the circle, to have a chance of a successful invasion.
I don't know if this is what you are doing, but its always worked for me.