I'm not sure about 1, but 3 and 4 are not (entirely) correct. They just released a massive chunk of completely new looking gear with the Abyssea expansions, all requiring new models and textures. And graphics could still be boosted for other systems, it would cost them the same as it would do with the PS2 (probably even less). Since graphics are handled client side, the PS2 could still render it with old graphics, it wouldn't change the game at all. It would simply look better on other systems, something that's already happening with menus. The question is, whether SE will invest their time to do this, as this would actually require a massive rehaul.
About the zone thing, I'm not sure, were the Abyssea zones just differently rendered DATs? I thought they had their own DAT files, which were just as big as the normal zones, or am I mistaken? I think the HDD issue was download related, although if that's happening, chances are the zone thing won't be far off either way.