They're rushing through a few chapters of the manga, presumably many of the LN.

It's actually a real thing: https://en.wikipedia.org/wiki/Sexual_dimorphism

As for Milim, I took it this way. They're not relieving tension. Rimuru is using brainpower to beat her. Everyone knows who she is. She is a power feared across the world. She's also not someone you could take down by force.

Rimuru is from another world, making things with recreated technology/culture from another world. How do you beat something so powerful it can do pretty much whatever it wants, whenever it wants, and no one will stop it if it sets its sights on something?

Bribe it with something it has never had before, and has no idea how to create. It's still a gamble. Milim could have threatened them for more. She's simple-minded, but not stupid. Rimuru lured Milim into mutual cooperation.

The problem though, is not it looks like their nation is one of her vassals. Now we've ventured into world politics.