FFXIV has always had seven hells/heavens. It was FFXI that had six. The out of universe reason was the Koji liked the sound of "Seven hells" as a minor expletive better then "Six hells" and came up with a lore justification for having seven hells.

The seven Shards that were Rejoined to the Source are gone. They flat out don't exist anymore. You can think of it like when two bubbles share a "wall" and when the "wall" is popped, the two bubbles turn into one big one. As far as we know, it's impossible to "undo" a Rejoining.

One of the reasons I don't like the idea of time travel is that it makes things like the Rejoinings that very, very messy. All the Rejoinings except the seventh Rejoining happened over one and a half thousand years ago. Preventing any of those would change history on the Source in such a massive way that I can't see any good reason why Eorzea as we know it would exist afterwards. Which would fly in the face of how the established overworld never really changes from expansion to expansion.