Sorry, but this is wrong, and this is why I'd reccomend you to catch up with the newer episodes of SU. You're missing out on the exact information which disproves your theory, but I'm gonna spoil it to you, you better watch it yourself.
I have another theory to consider. While so am not currently up to date with Steven Universe. The last one I saw was legs to homework. I think the mysterious gem is actually Steven himself. He is the heart of the Crystal gems after all. To explain my theory I believe Steven became corrupted some way and White used her power to save pink diamond as as a result accidentally separated Steven from his gem. So the movie will have the gems working together to create a gem for Steven aka the red heart gem we see in the poster.
I mean I DO feel bad for her too! (and Eclipsa since Solaria was her momma) but like both killed a lot of people, but would any of this had happened if the Mewmans hadn't arrived on THAT exact place in the first place? Not saying it was completely their fault, but like, whoo boi if they hadn't assumed the Monsters were dangerous maybe none of this would have happened?
I mean maybe that as well, BUT, wouldn't there be at least ONE mural left? Like the ones on the Moon? But there's only four. Even if it WAS a long time ago, there should be some evidence pointing towards that. Also the fact it's called The Heart of the Crystal Gems, I do wonder it doesn't have to do with the Diamonds, as much as with the Crystal Gems? But that's only my theory. But anyway, we'll see.
Well isn't that what it kind of stated in the show too? In Moon the Undaunted? and I know that's like VERY BAD thing to do, but it makes me wonder, if one of the earlier Queens had attacked against their families, and maybe even killed THEM? So it would be pretty much impossible for them to forgive the Mewmans. Obviously they could've gone with more peaceful method and be better than them, but maybe they just wanted their lands back, and for anyone who uses magic to suffer as they did?