Even when just starting out in Season of Defiance, one thing stood out to me; how accepting Mara is of us. Before and even now, while sometimes with good reason, she would keep secrets from us. The Dreaming City, Riven, Savathun’s containment, etc.
Now, she’s even lending us her paracausal powers. Not to even just us, but [i]all[/i] Guardians, as stated by the scepter. The scepter even states how weird of a move this is by Mara.
Was Mara compromised? Is this all in fact real and just recognizes the weight of the fight between us and the Witness? My guess is she definitely has a plan of some kind but we must wait until we can know more.
My theory is that she knows that helping us helps her keep the Witness away from her people. A ruler's first priority is the health and well-being of their people, and aiding the Vanguard in their war against the Witness will lessen, if not outright prevent, casualties among her own people. I think it's the same reason why we helped her and Petra deal with Skolas and the House of Wolves back in D1: to prevent them from becoming a threat to the City and its people. This may be a calculated move, but it's one where both the Vanguard and the Reefborn Awoken benefit. We get some much-needed assistance against the Witness, and they get some measure of protection and time to prepare for an assault should it become necessary to mobilize.