I've seen a picture of Adalind wearing a ring on Claire's Instagram. In that picture, she was looking upset
So I can only conclude, Adalind will agree to marry Renard just to get Diana back and even though her intentions are good, Nick will misunderstand her and this will create a huge rift between them, leaving a way for Juliette to come back into Nick's life.
When that happens, Adalind will become evil again.
I hope the Grimm writers haven't planned this because I really want Adalind to not lose Kelly or return to the dark side. And I don't want Juliette back with Nick.
So I can only conclude, Adalind will agree to marry Renard just to get Diana back and even though her intentions are good, Nick will misunderstand her and this will create a huge rift between them, leaving a way for Juliette to come back into Nick's life.
When that happens, Adalind will become evil again.
I hope the Grimm writers haven't planned this because I really want Adalind to not lose Kelly or return to the dark side. And I don't want Juliette back with Nick.