They did have a Plan B.
Plan A was to navigate traditionally by a combination of dead reckoning and Astro, with a mid-course correction augmented by sighting the pre-placed guardship Ontario.
Plan B was to DF to Howland/Itasca.
Plan C was to have the ship at Howland DF on them and give them a QDM.
There was no possibility of a diversion to alternate once they had reached where they thought Howland was.
Her last few transmissions make it abundantly clear that there was no possibility of a div to an alternate and quite certainly no attempt to fly to Saipan or Rabaul or Gardners Island.
The ugly truth is that Noonan cocked up his navigation. Contributary factors are likely to include hypoxia, dehydration and chronic fatigue.
The only realistic chance they had of diverting back towards the point of departure was if they had turned around after failing to find Ontario. Actually, her only chance of salvaging the project slipped away after she failed to identify the reasons why she was unable to get an RDF bearing on the Lae station during her test flight the day before departure.