Hi Jean,
1. The relationship N:1 should be made in Account, the primary entity should be Offer or Account?
This needs to be based on your business needs.
If an account corresponds to multiple offers, then the primary entity is the account.
If an offer corresponds to multiple accounts, then the primary entity is the offer.
Since the mapping is bidirectional, no matter which entity is the primary entity, the fields will be mapped when creating another entity from one entity.
2. Why for each new relationship, the Status for me is always "not managed"?
It is by design.
Unmanaged customizations are any change you have made to your organization through an unmanaged solution. The system solution defines what you can or can't customize by using managed properties. Publishers of managed solutions have the same ability to limit your ability to customize solution components that they add in their solution. You can customize any of the solution components that do not have managed properties that prevent you from customization them.