I think there's also another wrinkle here. The World Government knows that time is running out.
I think Imu sensing Joyboy's Haki at Egghead has basically put them at high alert. I think they KNOW that a huge war is almost inevitably coming and that they need to prepare.
With Harald, they could afford to string things along and take their time. They were confident that no one could challenge their rule. Not so much now. They NEED to secure Elbaph NOW. Before a war breaks out. And Loki is probably a key part of that.
Loki got a invitation now, because the World Government knows it doesn't have time to play too many games with this right now. (Ironic, given Imu's game-based powers). Getting Loki, and his supposed Legendary Devil Fruit, on their side would probably be a big win for their side. Loki might be one of the only beings who can stand toe-to-toe with the Nika Fruit.
Plus, Loki is a outcast from the other Giants anyway. So, it's probably easier to deal with just him, rather than having to fake respecting a entire country. Loki does not seem to care for his country and people in the same way that Harald did. Loki seems pretty selfish, even if he isn't a bad guy deep down. It's easier to deal with him, than it was with Harald, because Loki doesn't have the same moral hangups as Harald did.
Harald would definitely push back if he thought his people would be turned into a army for Imu. Loki...might not. From the World Government's perspective, Loki might just go along with it, just for revenge. This might be a misread of his character on the World Government's part. But, I could see the WG thinking that Loki has no reason to stay loyal to the people who locked him up for years.