...I think they probably wanted the 1000 slaves. They go through those like candy. They probably always need a pretty constant stream of slaves.
Just think about the Four Wolf Bridges, alone! Those are HUGE projects requiring TONS of people working on them constantly. And there's probably other projects like that all over the World outside the Grand Line. They would use those 1000 slaves pretty quick.
And part of it, I think, is kind of a "respect" thing. When you run most of the World, you need everyone to follow your rules, or else things pretty quickly descend into chaos. If you can't pay your taxes, you have to find some other way to pay up. You can't really go around making exceptions, because then all the OTHER countries will want a exemption from their taxes. And then the entire Government falls apart because you can't pay anyone.
And I don't see any reason Leuven would lie. I could see the World Government lying about their intentions. But, "good rulers" in One Piece rarely lie to their people, unless they're being directly blackmailed or controlled in some way. And Leuven didn't seem to be under anyone's control at that moment.
As for Imu wanting Shuri...maybe? But, maybe not? Imu does seem to keep pretty close to him most of the time. But, he's also letting her go out on missions? Which would be kind of strange if he desired her. Why would he let someone else borrow one of his favorite "toys"?
And comparing Shuri/Gunko to Vivi/Lili is one thing. But, it seems like the situation with Gunko is also a pretty similar situation with Bonney. And...they didn't seem to care about getting Bonney back unharmed. Saturn seemed pretty willing to kill her on Egghead. So, it might just be that Imu doesn't want Shuri or Bonney in play, outside of his control, rather than "desiring" her. I guess it could be a little bit of both?
I suppose it IS kind of telling that they didn't just Buster Call the entire island and just call it a day. Sending in a Gorosei, and a God's Knight, and foot-soldiers into Esperia does kind of make it seem like they wanted to capture something on this island.
But, I'm not sure it started out that way? If we extend what we've seen about how Imu operates during Elbaph to Esperia, I don't think the original plan was to kidnap Shuri. Following what we've seen on Elbaph, if Imu wanted Shuri, he'd send in a small group of God's Knight to sneak in and kidnap her before anyone knew about it. Starting a huge, loud war over Shuri just doesn't feel like it'd track with how Imu operates. A war would put Shuri at risk of being collateral damage, which if we assume Imu wants Shuri, he wouldn't want to risk.
So, I think the Government started attacking to assert their authority over Esperia, and THEN they found out something about why Shuri is special. THEN they decided they wanted to take her in, too. There was probably some unexpected escalation here. Imu probably didn't know about Shuri, until they were already attacking the island. At least, that'd be my guess right now.
We'll see how wrong I end up being when we hear the OTHER side of Brook's flashback, probably from Shuri/Gunko's perspective later on. Because there probably is more to all this, but maybe not in the way any of us are expecting.