Do you think Tywin approved of Jofferey & Sansa’s engagement? Or would he consider it a marriage beneath the lofty station of a Lannister heir? Following on that, in case Margeary didn’t exist/was not available/Lannisters managed to win the War without Tyrell help, do you think Tywin would still have allowed Joffrey to marry Sansa, or would he attempt to marry him to someone else(like Arianne) from a House that had not actively taken up arms against the Lannisters?

There’s nothing wrong with Joffrey as the Crown Prince marrying into the family of a Lord Paramount, that’s good dynastic politics. And even during the War of Five Kings, there’s value in getting a claim to Winterfell in the family – hence why Tywin moves quickly to make sure that a Lannister of some kind marries Sansa, both so that the Tyrells can’t and so that a child with Lannister blood will eventually inherit the North. 

So if the Tyrells hadn’t existed, I think Tywin would have been happy enough to see the marriage go through.