I do prefer option four though I had to check about lunar influences -- I wrote stories that mention the Moon but had to double check that it actually existed in the Narnia world. And from your preferred possibilities, I'd like to think the Telmarines had a different naming system that may had been adopted some time in the 1300 years between LWW and PC, and that everywhere else still used the old (Gregorian) names. Peter would have had to ask someone (Trumpkin? Cornelius? Caspian?) when was it they had arrived in Narnia at some point in order to dictate the challenge correctly.

"I see," said Scrubb. "Well now, let's get on. Tell us all about the lost Prince." Then an old owl, not Glimfeather, related the story.

About ten years ago, it appeared, when Rilian, the son of Caspian, was a very young knight, he rode with the Queen his mother on a May morning in the north parts of Narnia.

- Chapter 4, A Parliament of Owls, SC

With the inclusion of May by the Owls, I still think Greenroof as an additional month in the calendar probably makes the most sense. Who knows, the summer season in Narnia might be longer than our world.. especially after the Long Winter finally thawed out.