I am not sure whether one twin is supposed to live on the floor of the ocean or not. The maximum would presumably be a twin on the Earth and the other removed out into space. This has a relative boost factor of 1.0000000008, corresponding to about 0.024 seconds per year, giving a net difference over 75 years of 1.8 seconds. You spin it up using jets along the rim of the space station. You want to make sure that the jets fire evenly so there isn't any wobble.
Special relativity is a subset of general relativity.
The distance between two points on a geometry such as a surface is certainly going to depend on how that surface is shaped. The metric is a mathematical function that takes such effects into account when calculating distances between points. Those special locations don't become flat no matter how small a scale one considers. Certainly, it will have to be superseded by some theory of quantum gravity. We are not certain of the true nature of gravity because it must be quantizable, but we have no theory of quantum gravity.