Thanks for the reply, but I am afraid this is not solving my problem.
In parallel to this forum topic, I also opened a Use Case in the Customer Portal, which gave me the answer I had the suspicion I was going to get. I quote from the Use Case answer:
I apologize, there is not a way to view the activity through an API at this time
In fact, after your answer (and before the Use Case answer) I tried my script again, forcing some contacts to be deleted from the list and new ones, not previously in Constant Contact, to be added and I am still not getting any report or tags or whatever in the Contacts -> Activity section. I attach a screen capture of what I am getting.
So even though CC claims that with the new look & feel you can do everything that could be done before, I am afraid that is not correct, some functionality has been lost.