[The Squatter of Amon Rûdh]

After seeing the black rider, Frodo and Sam are uneasy and make more of an effort at concealment. "They kept now a stone's throw to the left of the road, and kept out of sight as much as they could." This is naturally harder going than walking on the road, so it's only natural that at a fork Frodo would choose the less travelled path as it offered more chance of concealment. Any pursuit would be more likely on the road to Stock.

After they fall in with Gildor, the company travel with him to "the woods on the hills above Woodhall", which leaves the whole company the following morning some half-way between the road and the Stockbrook. Pippin obviously expects them to head back to the road, but Frodo's plan is to cut directly across country to the ferry, as I suspect he had already intended before meeting Gildor. Pippin isn't to know what Frodo intends, though, and it's clear that he still isn't as concerned as his companions from the fact that he's still talking about pubs and beer. To him this is a simple walking party, whereas Frodo and Sam - who are older and wiser - have begun to realise that they're in danger. Hence the only mistake is Pippin's of imagining that Frodo would decide to rejoin the road so that he could follow the CAMRA trail.

The map of the Shire is, indeed, the work of Christopher Tolkien. In the first edition you can see his initials at the bottom left of its title box. This is not a sign of potential inaccuracy: CRT has a good eye for inconsistencies and was no doubt asked to draw the maps because he's good at it. There was plenty of time between the completion of the LR narrative and publication (a manuscript version was in existence in 1949) for father and son to talk at length about all the maps to be included. While there may be some mistakes, completely misdirecting an entire road would be a big enough one to warrant paternal admonishment.