Regarding Laurie and whether she manipulated Shane:
Not a fan of Laurie, but I don't think she talked to Shane with the intention of making him unstable (or even more unstable), since if that was the case, what if Shane does suceed in killing Rick or doing something "bad" out of "obsession" over Laurie? But of course, maybe Laurie IS that dumb and manipulative that she's willing to put her family at direct risk.
Regarding Shane and his intention for Rick:
I think he had a lot going on in his mind but nothing solid, because he was confused because Rick is still / was his friend:
-lead rick to the bad guys and see what happens and so be it
-lead rick to walkers and in chaos maybe he'll (shane) submerge as the only survivor
-lead rick to the woods and maybe something will happen and they end up trying to kill each other, and he (shane) survives.
But the fact that the faceoff took place so friggen close to the farm (close enough to the farm for Carl to book it over, and close enough for the viewers to see that they were out of the forest and close to the farm), makes me think that Shane wasn't able to come to any conclusions as to what to do, but since Rick confronted him, he decided that it was "as good as anyplace" to do the faceoff. In conclusion, imo, he had a lot of loose plans but wasn't able to commit to anything out of his clear confusion and inner struggle, and Ricks confrontation gave him the momentum to "accept" the "accusations".