The series was originally intended to run a year and a half, yes, but movie 1 was not intended to be the series finale. It was always meant to be screened the July date it did (which caused complications after the show went on hiatus a while, since as a result the movie tie-in episodes aired long after the movie had been in theaters), roughly 1 year and 2.5 months into the series.
The movie was not rewritten to any particular degree either, since it's an hour-long cartoon, and these take time to produce. You can't just go in an rewrite them halfway into production. It's impossible. And of course, there are no reasons to assume any such rewriting took place aside from "I always heard some stupid rumor state so".
As for that trailer... it's not meant to represent some version of the movie that was thrown away and replaced with the one you got. It's from 1998, meaning it was screened less than 7 months before the movie premiered - as mentioned above, that's wayyy too late to do any extensive rewriting.
Also, there's no plot to the trailer. It's not something that's meant to be extensively examined and theorized about, it's just some pictures with a song set to it, telling the audience there's a Pokemon movie coming.
As for the girl... the song used in the trailer is "Pocket ni fantasy", the current ending theme to the TV series at the time it (likely) screened. The trailer itself fits the lyrics, so you might consider it a music video of sorts. The song is sung by Sachiko Kobayashi and Juri Ihata, portraying a kindergarten teacher and a little girl. Kobayashi played Voyager in the fist movie (and also sung its ending theme), making it likely the girl in the trailer is meant to portray Ihata. That's the logical conclusion to make from it, at least.