The purpose was ostensibly not to rescue Lea but Nemesis within Lea. There may have been other purposes, but I am not sure Mab didn’t allow it to a point to flush out Nameless, which wouldn’t affect any hypothetical link to Demonreach as the Warden was allowing this, as with Hades Vault.
We see the story from Harry’s view and he is an unreliable narrator who only sees part of the picture, Mab definitely sees more, but we see nothing from her POV. Skin Game was a revelation because we got to see more of her larger plan and PT/BG just confirmed this, providing a modus operandi, we have to therefore reconsider what Harry has seen in the past in light of that. It’s not a recon more of a secret history, Jim has only shown us what he wanted to show us, but certain things have been there all along and not added subsequently.
Arctis Tor may be very different to what Harry thought it was. The Law indicates that. Harry and his party may have been manipulated to provide a valid reason to watching third parties including Nameless to recall Winters forces. The Phobophages were after all part of Winter and subject to Mab’s control. Part may also to give Harry certain information i.e. Hellfire being used to pass onto Nick, if you think about Winters involvement in SF.
Mab is not arbitrary and does nothing without a reason. She plans well ahead and thinks nothing of exposing a flank to draw an enemy out or using mortals like Molly or Harry in her schemes, even if it might be fatal to them. She will sacrifice her own children for her schemes.