Hey @FSaulier, good question!
When replying to an email, you do have the option to add/remove recipient contacts and cc/bcc contacts (more on that in this HubSpot Knowledge Base article). In the scnario you mentioned, you should be able to reply to the most recent email in the thread and edit the recipients as needed.
That said, I totally understand the frustration with not being able to reply to a previous email other than the most recent one. I encourage you to share this thought on the HubSpot Ideas forum to see if any of users are having the same challenge!
Hope this helps!!