Who's talking about datamining here? Don't the words of Sakurai mean more than that?A lot of this speculation really depends on how long these characters were held out on. When Smash released, they were only developing two characters based on the datamine, which was Joker and Hero. Plant wasn't released yet at that point but it was clear he was a holdover from the base roster that was ostensibly 95% complete when they were ready to show him.
Considering the datamine was largely obtained from pre-launch day versions it's safe to say that said parameters were in the game around the time the game would've gone gold, which suggests Joker and Hero were in development since around November, so they alternate developing two characters while putting the third in pre-production during what's essentially the debug phase/holding pattern for a finished character. That's still extremely early for any information on who the first post-FP character is imo, as I'd assume they'll only really go into full production on that in November.
They were working on three (likely at varying levels of priority; with only preliminary work being done on B&K at this time) at launch.