The primary source is Earhart herself : when she says to fly a position line heading 157-337 compass , she admits that the one line approach has been executed , especially when communicating "we must be on you". Besides this , a direct approach to so small an island when coming from a 10 hrs. plus nightflight was for practice suicidal , uniformly notified in textbooks.
This is only one of many possible interpretations of this transmission. Unfortunately, this is also the least likely interpretation as well. As Ric has already said, there is nothing to indicate any intent of using an offset course. Indeed, past practices in and of themselves argue against it.
Clipper Route SOP was to navigate direct ( or as direct as was possible ) until in the vicinity, and then at that point ADF was used to 'home' on the destination. PanAm built large high powered transmitting stations at all of their ports-of-call for just this purpose.
You appear to forget that this was also the intent when it came to finding Howland. This flight was no different from the scores of Clipper Routes that Noonan had personally laid out and flown. There was never any plan for Noonan to be the sole source of navigation to Howland....only to (once again) get them into the neighbourhood and then let the ADF take them in.
A more or less workable plan, when everything functions properly. When it doesn't....well, we've seen what happens then.
No...for better or worse, there is no evidence that I have seen to indicate an offset route. Indeed, when one is planning to home an ADF/NDB signal the last several miles, an offset would be more a hinderance than help. Not to mention that doing so would also require more fuel....not a sterling plan when you're already offloading items to reduce weight and fuel consumption in the first place.
....TB