The "narrow footpath" is also suggestive of the narrow-mindedness of the Puritans and their strict religious beliefs. You may have heard the reference to "walking the straight and narrow". There is no room for straying from the path of Puritan beliefs, and they are very unforgiving of those who do stray. From the time the people discover Hester's pregnancy, they are judgmental and unforgiving of her sin, forcing her to wear the scarlet A as a reminder of it.
Ah, good question. There is a long tradition in Christianity (and indeed, in other religions) of treating one's passage through life as a journey, and the metaphorical direction one takes as a path. In many of these Christian examples, such as Dante's Divine Comedy, individuals lose their way and/or pass through dark and tangled ways. This is a threat to them, and especially to their souls.
The forest represents wildness, but especially, spiritual threat for Hester, and the narrow path through it represents the narrow path to moral life and salvation.
You'll notice she guides Pearl on the path, and encounters....Dimmesdale.