I've just watched this movie. It was really fun and the battles were mostly good, although I was a bit annoyed at times (ZX got shafted again, just like in RX...). It's easily among the best Rider movies.
The biggest problems were some badly done scene cuts and the pacing, which was way too fast at times (ShadowMoon is in the tower... 5 seconds later, he's in the battlefield without any kind of transition), but that's acceptable considering the running time of the movie.
Random note... what was up with the random Ryuuki quote? Every Heisei Rider that lacked the original actor seemed to just yell or stay silent, but Ryuuki got a random full word during the battle...?:sweat:
The Black RX we saw in the show was definitely not the original, as he had no past connections with Golgom or the original Kingstone powers. But otherwise I get your idea
That's not really confirmed. There's some evidence that he never was Black (he said "another new enemy!" when the Gorgon monster appeared), but it's nothing conclusive.
However, there is also some rather strong evidence that he was Black in the past. Tsukasa got a Black card when he went to the RX World after all (and it was blacked out, not gray, just like the Hibiki card after Hibiki became Gyuki).
About Den-O World/Alternate... The Momotaros in this movie didn't seem to know about the Final Formride, wondering about what was happening when Decade used it on him. So, isn't that evidence that he isn't the same Momotaros that Tsukasa met while traveling?