A newsletter signup that writes to a spreadsheet is a newsletter you will never send, because exporting and importing is one job too many.
Connect Mailchimp and the signup goes to the list you already have.
The value is in the tags. Knowing which page somebody signed up from is what makes the first email worth opening, and it is exactly the detail that gets lost in a manual export.
A standard API key. It is account-wide, so use the seat with the least access that works.
No. It adds people to a list; sending stays in Mailchimp.
Yes. Connectors are part of the plan; there is no separate integration fee.
At any time, from the same panel. The credential is deleted at the broker.