For point 1, is the depth of the ocean significant compared to the distances traversed? My back-of-the-envelope math suggests it’s less than half a percent for a cable from New York to England. (6 miles down + 6 miles up, divided by rough great circle distance of 2600 nautical miles.)

I would think a bigger factor would be that the cables (IIRC) don't go in straight lines (which you did allude to).