Apocalypse Maiden: Daido intends to use Phillip as a conduit to activate NEVER's X-Bicker.
Backup from Otherworld: the spirit of Sokichi Narumi inexplicably appears at the Detective Agency to give the Lost Driver to Shotaro.
Chekhov's Gun: at the beginning of the film, right after Eternal sends the T2 Memories raining down on Futo, Shotaro gets irritated because of a hole in the Agency's ceiling. It so happens that a T2 Memory actually landed there, creating that hole, and at the time the Double gang never knew.
Clap Your Hands If You Believe: Shouts of encouragement from Fuuto's citizens somehow summons a powerful gust of wind to gather around Fuuto Tower, transforming Double into Gold Xtreme Mode in order to save the day.
Nice Job Fixing It, Villain: the Heat Dopant attacks Shotaro, thinking that he might attack NEVER headquarters at Fuuto Tower because Phillip is captive there, except Shotaro knew nothing about those two details until she actually told him. Oops.
Phillip would have never found the means of disabling the Zone Maximum Drive, returning power to the T1 Memories, had Eternal not hooked him up to the X-Bicker device.
The Worf Effect: FangJoker, established as Double's second-strongest form, getting easily trounced by Eternal.
CycloneJokerXtreme, the strongest form not debuting in this film didn't fare much better in it's first fight with Eternal and that was the form they were almost defeated in during the final fight.