I think everyone is blaming the wrong people. The success and failures of this game are mostly a joint venture but if anyone caused the snowball it was Toriyama. Because he is one of the higher board members he got to make his game, FFXIII take precedent over Versus. Originally they were being made at the same time. FFXIII hit so much trouble that they started pulling Nomura's team onto it. One of the most damning things for Versus was when the already complicated process of making an engine had to turn to focus on a less complex game. Crystal tools should have been made primarily for Versus first, then XIII, that would have made the transistion simple. I swear I remember hearing that at one point Versus was ahead of XIII. Yet after FFXIII released they were left with the question on how to transition that same engine to work with a much broader game.
Nomura suffers from being too artistic and creative for his own good. I will say that. Much like Kojima..... he needs a handler to get him to scale back. I think even if the engine had not impeded him so much he might had over promised. It sucks because he kept having to be stolen away from his own projects to help with others. He is probably the most worked of all the Square Enix creators. Remember.... Square snuck in 2+ extra games into their timeline of the FF Versus XIII release. XIII-2 and Lightening Returns added to the derailment. I honestly think that once Nomura saw he was not going to be able to make the game that he promised, he no longer was up for it.
I distinctly remember leaks starting to happen the year before this console gen. They said that Square had planned to quietly scrap Versus after years of delays. Problem was....once that leak happened SquareEnix Stocks plummeted. They quickly put out a statement that the game was still in development. I think the game WAS actually cancelled, and everyone involved was ready to move on. Yet the Shareholders recognized FF Versus as the most appealing looking entity they had revealed and the hype for such a game would never just go away quietly. Many a share was bought on the premise of this game afterall. So at that time they kicked back into overdrive, but now they were given a time limit...... FF Versus XIII, now FFXV had to come out else so many deals were about to be broken.
This is where Tabata got tapped in. He was one of the only SE devs churning out his projects in a reasonable time, many of which were well received. He came in as the fixer for this game. Yet......I think some of his liberties ruined the spirit of what the game was promised to be, even with taking in consideration . Mainly....Luna. Why? One of the most appealing things of the original Versus concept was Stella, a badass Rapier wielding counterpart to Noctis. The removal of Magic was also damning and the limited level of control of the party members when they were previously stated to be uniquely playable was unfortunate.
Final Fantasy XV could have a movie written about it, it's development had so many turns.
Oh, there is no doubt that Toriyama is the worst of Square's directors, though the changes that Tabata made to XV disappointed me (mainly the same changes you just mentioned, dropping Stella in favor of the much blander Luna, dropping the proper magic system for that crappy one, dropping full player party control, story changes, etc.), I still like Tabata alot more than Toriyama, I'm glad that Square basically demoted him from AAA development to mobile.
And I agree with you that Nomura is one of those visionary type game directors like Peter Molynuex and Kojima that needs somebody to reign him in, or else his projects just keep getting bigger and more ambitious to the point where they are no longer practical. That was definitely a problem with Versus 13, he promised things that simply wouldn't have been possible on PS3, which is one of the reasons why it's development was so slow, they were just waiting around on new tech for alot of that time.
What I want to know is what the heck Ito is up to these days. The man has been pretty much off the grid for the last decade, his list of game credits these past 10 years have been so sad to see, 2 mobile games, special thanks on a few games, and supervisor on FF12 HD, that is it. Back in 2013 Hashimoto said that Ito was preparing some new projects, but we've heard nothing about these projects since then. I honestly believe that Ito is their best bet for directing FF16 now, Nomura is doing 7 Remake so he can't do 16, Kitase got promoted so he can't direct it, Tabata is gone now, Toriyama is awful, seems like Ito is their only hope of making a good FF16 at this point.