Depends on your region as the U.S. traditionally gets the games anywhere between September and December for Holiday season. Japan gets them around March through May to coincide with Golden Week and PAL gets them whenever SE decides to stop screwing with them, which is usually late Spring, early Summer. They definitely try to release them around holidays known for present giving.
I doubt we have to worry about anything as KHIII is probably still two years off and we can only guess how far the VII development is. I know fanboys are thinking 2017, but I feel 2023 is a safer bet, especially if that teaser trailer is all they've managed to do so far. SE has yet to really prove they can get a large budget AAA title on the market within a reasonable time, so we'll have to see how quickly they release XV, KHIII, the new SaGa title no one outside of Japan cares about, and probably Project Setsuna before we can get a good idea of when the VII Remake will come out.I don't think it will affect the other games development as they are made by different teams like you mentioned. It's more about making SE's books look good, they will want each of those games to release in a different financial year. Even if KH 3 and the VII Remake were completed on the same day they would delay one into the next year for that reason alone. You see other Dev's do this quite often.