I love how you added this game mode non-destructively to the game while preserving the existing game. That certainly must have added a level of challenge to your work this round. But it's good that your existing game made such a solid foundation- I can tell why this game is a community favorite and I'm sure I'm not alone in hoping that you bring it home for a complete release on steam.
Now, for the criteria:
Was it fun? YES!
Did it include the core elements from the original? Absolutely! The only thing that was possibly missing was the ability to rescue my guys after they die but I might have missed that.
Was it polished? Damn straight!
Audio & Sound? The music was awesome and the audio was super responsive. I loved the SFX for the grubs, it sounds like the noises my youngest cat makes when she's hungry for kibble!
How well implemented was the UI? Honestly, there is a ton of UI here and it's all super deliberate and well-made, complete with animated transitions and everything. The only way I could see it improving is with a bit more sound and some hover effects on some of the elements (but that's super nit-picky)
Minor Criticism: I would have loved to see an option to switch between ortho and perspective camera while in the Gain Grub gamemode!