Ivanhobe
The Extraordinary Fan(boy)
Also, the guy with one arm is introduced and leaves in about 2 minutes, acting like we should know him, but he had never been mentioned at all before.
That bit requiered a bit Rider knowledge, the name of the character is Yuuki Jouji (played by Gackt), he is the alter ego of Riderman, the fourth Kamen Rider as he appeared in KR V3. I will not go into too many spoilers, but Yuuki Jouiji is a legendary character, in his original story he worked for as the leader of the Science division of Destron until he was framed by Marshall Armor who branded him a traitor and marked him for execution by dissolving him in acid, but his comrades betrayed Destron and helped him escape, though not before his arm was dissolved. After that they operated on him, replaced his arm and he becoma Riderman to exact his revenge on Marshall Armor.
In this movie they basically took his original story and merged it with Decade´s backstory, which is actually really cool because it might imply that this alternate version of Yuuki Jouji was the creator of the DecaDriver and the DienDriver.
1. Is this in continuity?
This is actually a complicated matter, All Riders vs. Dai Shocker was meant to be the original ending of the series, the events of the movie are foreshadowed many times during the series, but those plans were scrapped and we got Movie War 2010. According to the fine gentlemens of Toei, this movie IS canon with the series (even MW2012 references it), the problem is that given the way the show works there is no way to fit it in the continuity, so the movie is canon and non-canon at the same time, we just have to play pretend.