This is all very helpful, but I'm still confused as to why the original nether portal in the Overworld rebuilds itself.
Portal A "was" at say 0,0, which we completely destroyed. The entrance to the netherworld is beside the nether temple.
Portal B "is" at 1000,0. The entrance to the netherworld is still beside the nether temple.
We enter portal B & when we come back to the overworld we are teleported to portal A's location AND portal A has been completely rebuilt.
Just for experimentation I will travel an additional 1000m's and construct yet another portal just to see if this behavior continues.