15 letters 1916 Frost verse ... or, diverged: Start of a saying about meaning well Answer: THEROADNOTTAKEN
15 letters Driving test challenge ... or a hint to this puzzle's circled letters Answer: PARALLELPARKING
15 letters Frances Hodgson Burnett kid-lit novel ... and a hint to 21-, 26- and 45-Across Answer: THESECRETGARDEN
15 letters Fancy-schmancy language ... or the contents of some special squares in this puzzle Answer: FIVEDOLLARWORDS