... do we have any documentation on some contingency plan discussed by Earhart and Putnam in case she did not find Howland ...
The short answer is no, there is no known documentation of a contingency plan. Of course, turning back if things are not going well is always an option if you make the call before you reach the Point of No Return (after which you don't have enough fuel to get back). They did that on June 18 when they tried to fly from Akyab to Rangoon but had to turn back due to heavy rain. (Last Flight page 197). On the Lae/Howland leg they clearly continued past the Point of No Return so things must have been going okay so far.
There is some anecdotal evidence (the least reliable kind) suggesting that Noonan may have had a Plan B. "Fuzz" Furman, an American who spent time with Noonan in Bandoeng, Java, June 21-26 remembered that Fred was very anxious about finding Howland, and yet Chater in Lae reports that Earhart and Noonan both felt confident at the time of their departure on July 2. What changed? The sunrise LOP. When he was in Java, Noonan expected to make the Lae/Howland flight several days sooner than they actually did. He didn't expect the sunrise LOP advanced through Howland to be any help in finding an alternative island. But in Lae, when he knew they would depart on July 2, he could see that the sunrise LOP would be 157° -337°. Advanced through Howland, the line also fell through or close to Baker, McKean and Gardner. It seems like that would be reassuring and may account for his apparent change in his confidence level.
AE and FN's Plan B is a perennial question. It oughta be in a FAQ. I don't have time to check just now whether it is there already. Search for "plan B," Vidal, Gilberts, etc.
Yeah, we probably need a FAQ. As I recall, during the search Vidal told somebody (the Chief of Naval Operations?) that AE had told him that if she couldn't find Howland she would turn back to the Gilberts and land on a beach. Of course it's a totally impractical plan (she was good at coming up with totally impractical plans) and, besides, she wasn't the navigator.
Anybody have the details about the story at their fingertips?