It's the third ending theme, Fantasy in My Pocket, which premiered with the Porygon episode on December 16th, 1997. The Pokémon anime wouldn't return until April 16th, 1998, moving from its prior Tuesday timeslot to Thursdays.
https://www.youtube.com/watch?v=aavLVne9AQ8There was a special TV discourse over what happened with the episode, and it came back with an original episode that wasn't part of any of the prior listings (Pikachu's Goodbye).
The two original characters are Voyager from the first movie (dub: Miranda, played by Kobayashi Sachiko of the Pokémon music group Suzukisan, who did a number of songs for the early franchise) and Ihata Juri, who plays the little girl in the song. Suzukisan also has Raymond, an American living in Japan who plays the character of the same name from the beginning of the movie, the guy with the Donphan who battles Ash. The third member is Imakuni?, who appears in a number of cards for the TCG, and in the GBC game about the TCG.
The trailer itself is advertising that Mewtwo Strikes Back and Pikachu's Summer Vacation will be shown at the July Summer Break Road Show in TOHO theaters. There's a warning at the bottom mentioning that it contains intense visuals and animation, likely as a precautionary warning after what happened with the Porygon episode, so based on all of this, the trailer first likely aired between April and June of 1998, while the first movie premiered July 18th.
We know a number of episodes that were supposed to air from December 23rd, 1997 to March 10th, 1998 (some tied in with the dates themselves by featuring the holidays), but everything got rearranged. But the anime lost 10 weeks of episodes that could be used to build up to the movie (it had two double weeks), as it should have been premiering around the time of episode 63 when Gary battles Giovanni, with subsequent weeks teasing Mewtwo. Certain events (ie. Charmander's rapid evolutions) were likely intended to be more spaced out in the schedule.
