Yes this is a very frustrating problem. Just follow the steps in my post (linked above) and you can get this working. I spent hours running into brick walls and you don’t have to do the same.
To stop Webflow changing the field names, set the name on the Webflow component that wraps the checkbox and its label. The name is inherited by the children. Details on this are in my post. I don’t think the behaviour you are describing is a bug, it means that checkbox group elements maintain the same name to ensure normal form behaviour.
The bigger issue with making Mailchimp groups work with Webflow forms is actually nothing to do with Webflow, rather how Mailchimp handles form submissions outside of their default forms. When a Webflow form is submitted to their URL, the field values are appended to the POST URL in the query string (nothing gets sent in the body). From my testing I could see that the URL was being formed properly, and unticked checkboxes were being sent as
false values for their corresponding fields, as they should.
However Mailchimp seems to ignore this, and if a field name is present in the request then it treats it as true. So when I submitted a form with 1 of 3 group checkboxes checked Mailchimp would add me to all 3 groups.
The only way I found around this was to create a separate ajax request attached to the form submit event. And to only send field names for checkboxes that were ticked.