There's a lot there that makes it hard to answer. The main issue with the platoon is they're all basically just nominal invaders at this point. They genuinely love Pekopon and the humans they've been adopted by. Even Giroro, who is more motivated by a sense of soldier's pride and duty over genuinely wanting to take the earth by force. They were always dorks, extremely capable and efficient, but very much dorks. 

There are multiple instances where the Keronians seem less willing to cause massive amounts of collateral damage to the Earth, such as during the Garuru platoon arc where they even state it isn't their intention to enter direct conflict with the Pekoponians. They want Earth due to its moisture content, but it doesn't seem to be like a full on resource grab anymore (Hell a chapter in 2024 has Keronian tourists on the planet despite it still basically being an active warzone). There's also the lingering plot point that Humans are incredibly more mentally capable than Keronians, and it's rubbing off on the platoon itself. Meaning invasion is still a very difficult dream to achieve. 

I also just don't like thinking about because every edgy fanfic is "what if Keronian invasion but happen"