"if a house floods, the concentration of water vapor upon the bottom side of a tabletop will cause the bottom to swell which in turn causes cupping on the top."
Let me tweak that a bit. The reason a lot of tables will cup upwards in a situation like that is that, against all the standard advice, most tables get a much lighter finish on the bottom than on the top (if any at all). It's not that there's more water vapor closer to the floor, or anything like that; it's that the lightly-finished or unfinished bottom surface absorbs water more readily than the heavily finished top side.
"Assuming normal conditions (no atypical spot concentration of moisture or heat) and assuming that the lumber is dried to 6-8%, can I then expect it to remain fairly stable from that point on?"
No, not really. Wood moves whenever its moisture content changes, and that happens whenever there's a significant change in relative humidity. This process of expansion and contraction with environmental changes does not stop because the wood was once dried to 6% MC. What you accomplish by properly drying the wood to as low an MC as it's ever likely to see again is that joinery is more likely to stay tight, and you eliminate the possibility that the wood will shrink dramatically as it acclimates to its new indoor environment. I met a guy once who lived in Denver, and who ordered a custom-made table from a shop located in some very humid area. Evidently the wood was not fully dry, because the table was circular when he got it, but within a few months was visibly oval.
"And if I understand you correctly, whether that lumber has been seamed in 3-inch or 8-inch or 16-inch planks is not so important, as long as that lumber was properly dried?"
As long as it was properly dried, and as long as it's then not exposed to extreme conditions like being stored in a damp basement, or a flood or exposure to extreme heat... then, yes, the plank width isn't so important.
The objective of where all this is leading is that I want to do more with wide planks and maybe even live-edge but I want to be sure i do it correctly and to prevent cracks if at all possible. Because wwe ship nationwide, EMC/climate varies a lot.
Shipping finished, wide, thick planks nationwide is a bit of a tall order. Average humidity in, say, Denver, compared to Baton Rouge, is going to be very different, and it will have an effect on large pieces of wood. You should read Hoadley's "Understanding Wood." Also, get a good moisture meter and learn to use it. You can't prevent wood movement, but you can anticipate it and design furniture so that it can move without damaging itself.
"What are the pros and cons of edge-grain-up as opposed to plank-style for stability of edge-glued-panel tabletops?"
To a tree, there's not really such a thing as "edge grain." It's all just wood. You're describing butcher block, which is a way to build up thick, heavy tabletops from thinner, less-expensive lumber. It does help with stability, in an extreme version of the way that gluing up a top from narrow planks does, by randomizing the orientation of the annual rings so that if one piece warps in one direction, it's countered by a neighbor trying to warp in the opposite direction, so that the overall surface stays fairly flat.
"- How does Iroko compare to Walnut for stability?"
I've never worked with Iroko, but just going by the numbers I'm finding, my guess is that Iroko is a little less stable than walnut, but not terrible.