I thought the narrative purpose of the green haired folk was obvious. They were to contrast Bella and Kane. Kane never listened to Bella and learned her lesson too late, while the other guy made more of an effort with his kid before it was too late. It was a secondary moral lesson to the Climate Change lesson of the main plot.
I thought it was decent enough overall, and reminded me of old Who in concept, minus the lack of a TARDIS for most of it. (TARDIS would have let them solve the problem of getting off planet right away though so they wrote it out the easy way.) Doctor and Companion(s) land in seemingly mundane area, life-threatening situation appears, Companion(s) run for their life while the Doctor puzzles out a solution. Problem is solved one way or the other after gratuitous deaths, and Doctor and crew are on their way again. "The Ark in Space" is one that comes to mind as an example of this sort of thing, but if you want a modern era example the one where the Doctor ended up on that medical ship with the war hero is another example of this sort of story.
If you're going to ask why the Doctor doesn't use the TARDIS to go back to immediately after they left to rescue the last two, well that sort of thing happens a lot. Why doesn't the Doctor go back to stop the problem before it's a problem? I know they use the "Fixed points in space and time" as an excuse and issues with the Doctor doubling back on her own timeline, but that's going to be a problem with -any- time-travelling plot.That's something you just have to learn to live with.
Now all that aside, the Benni kidnapping was rather contrived, as the Dreg had no way of knowing he would be suitable bait to draw the others out., and they certainly didn't drag any of the others out to "play" with them. It had to happen for the sake of the story, but there was likely some better way that could have been used, I'm sure. I can accept the finishing him off off-screenhowever, as while it got back the classic jump-scare the kids behind the couch that Who is known for, showing him in the state he was likely in at that point would probably have been too graphic for the show.
Hyphen's makeup was a bit too simple for modern Who as well, but would likely have fit right in in a Classic serial. And yeah, she did remind me of Barf.