Wow, it almost totally works. I plucked the Group selection DIV from the embedded form and stuck it inside the Webflow form as an embedded component (right before the Submit button), and the data does indeed get sent through.
Trouble is, it somehow registers ALL the group options as checked, even though the embedded code I inserted clearly only has one option checked. I know that usually works because if you put the entirety of MailChimp's embedded form code into a separate Container in Webflow, it functions correctly and only has one option checked (which consequently manifests itself as the only group assigned to that user in MailChimp).
For some reason, separating the specific Groups/Segments DIV from the rest of that code causes Webflow to consider all of the options checked (which manifests itself as ALL groups being assigned to that user in MailChimp).
Check it out:
Public Page: http://gamecake.webflow.io/emojicharades
Read Only Designer: https://preview.webflow.com/preview/gamecake?preview=d50fe8c3b0d4a964c75c7a6ce3e3b64e
Here is the entirety of the MailChimp embedded form that I am plucking the specific DIV from: https://pastebin.com/ctfvTNS3