Yes, you're right, you could get away with walking alone, even riding in a carriage alone in some circumstances. This does deseve futher explanation. First I just want to say that Lizzy, as we know, was not consciously trying to break this rule. She informed Darcy of where she walked so he could avoid her. Instead he used the information to accidentally-on-purpose meet up with her. They were probably at the limits of permissiveness because these walks seemed to happen regularly and in the remote countryside of which no one else was aware. Wentworth and Louisa's short walk alone to glean nuts in the hedgerows seemed okay.

In Marianne's case, we don't hear of any walking alone that I can recall, but her one ride was way beyond the bounds of propriety because, not only did they ride off alone, but they rode to Allenham and went around the grounds and home under inappropriate circumstances.

My opinion is that a rule broken here or there could be gotten away with in trivial or 'accepted' situations. A small non-intimate gift such as a found bird's feather handed to a young lady would probably pass. The flowers that Marianne receives when she is sick in the movie might fly under the radar. But expensive things such as horses or pianofortes or intimate things like a necklace were definite no-no's. Marianne broke all these rules, the letter writing several times over, and was very unconcerned about it or any possible consequences for herself or her family. Who knows what might have happened to her reputation if fate had not intervened. As it is, everyone thinks Willoughby broke the engagement and all of Marianne's actions were those of an engaged young lady. When Elinor tried to give Mrs. Jennings an honest assesment of the relationship, Mrs. Jennings refused to believe it because she probably coudn't fathom someone so completely ignoring the rules to the extent that Marianne did without being engaged.

Basically, I think good sense and judgement had to be used when treading the minefield of courtship. Mainly, it was prudent to stay within society's norm. Gossip was an enjoyable passtime for many and new things to gossip about were constantly looked for; it didn't take much to lift an eyebrow and get tongues wagging.

Thanks for the qualification on walking. :-)