Oh my. Are we sure this isn't a plot to get Lydia as a replacement for her father by Mab? That would be an appropriate revenge from her, I feel, especially if Lydia being was only possible because of Ankou slipping the leash. Also Mab standoff. Shame we don't have "kill immortals" charm yet.
On the given options... Dealing with Lasciel is a bad idea. Like, totally bad. And Mab would probably know, and it would still be a breach of Harry's word. That's probably a no go. Calling Mab now wouldn't fulfill the oath - Harry still hasn't found elder Ankou.
I'll point out that Mab, as a once-human, might be susceptible to the Black Feast. So a Galaxy Brain move would be to lure her into the ritual and feed her to Molly / Lydia / Gard. That's totally a stupid idea, though.
We could just leave Harry here, so he could continue his search for Ankou on his own, while we deal with Holt and find Ankou via the Crown, which should be faster than Harry's methods.
Also, "find" doesn't mean "be able to get to. So, find Ankou without Harry. Warn him, so he can escape, then give Harry his location, and let him summon Mab.