Okay. Friendship is Magic and all, but, Spinel, get your ass back here. You've got some questions to answer.
First of all, where did you get that Rejuvenator? Now, I'm willing to let my imagination fill this one in, and assume it just happened to be left in the Garden. The implications of which are a little bit haunting itself. I wouldn't put it past Pink to have "reset" Spinel a time or two or any time she got bored or thought it would be fun.
The Injector, tho. Where the hell did that come from? Seems kinda new-tech to me. That doesn't look like the sort of thing Pink would leave lying around "just in case." So, somehow, she manages to get her hands on a possibly-new Injector, filled with prodigiously potent planet poison, conveniently capped with a gem in her own image (a special gem that seemingly nobody in recent memory will have ever known about), all within the span of... like, a day, between when she gets the gem broadcast message and arriving on Earth, all without the Diamonds ever knowing (you'd think they would notice something like that leaving Homeworld).
I don't actually expect to see that one explained. But if they did, I guess they could set up somebody somewhere out there that still holds a grudge, giving Spinel the means to carry out their plan without direct involvement. Still, bit of a... stretch. Ha.
Overall, kinda didn't really like the musical nature of the movie. It's not exactly unusual for the franchise, but, the songs used to be a little more special. Packing 80% of the movie with them just didn't leave much room for anything else. Some history/exposition was nice. Mostly, the whole thing felt a bit rushed. Was ok with the quick villain setup, no real need to do a lot of build up for that with the show/setting fairly established. But, it just kept rushing along. I blame the musical-ness. But also I guess a little bit of setting up a conflict that isn't so traditional. They did all the Epic Fight stuff back on Homeworld. In a way, I suppose looking at the entire movie as setting up for Spinel's redemption, instead of a conflict to be resolved by Steven/Gems, kinda helps with that. Idk. Conclusion, I wasn't entirely thrilled with how it turned out. But, it also did pretty much fit with the show. Can't really complain all that much.
However, I did find it oddly poetic, how much of the plot you can sorta trace back to this song. Evidently, judging from the recent comments, I'm not alone in thinking this. It's all coming full circle. I genuinely expected Spinel to start singing some variation of this in the Garden. Maybe there are some music nerds out there that will dissect it enough to find some similarities.