Good theory, not exactly sold on the use of Poneglyphs as literal weapons or Kid's DF controlling them. Kuma's DF awakening is probably the repulsion of intangible things, the reverse effect would be good though too kind of like Pain in Naruto.
As much as the Red Line is theorized to be destroyed, I don't think that was created by someone after AK fell unless there is a super mythical paramecia capable of doing something like creating something so big. I don't know what exactly is the height (from deep down in the sea to the top) but I think only a part of it has been altered to separate the seas. We do have creatures with tremendous power like the Sea Kings and Zunesha but the Red Line even dwarfs them I think.
The Lunarians I think used to live above the Red Line so the Red Line existed even before AK fell and is most likely a natural phenomenon.