Nephi uses “and it came to pass” to show a movement in narrative time. The phrase emphasizes an event, and shows it to be in a temporal sequence after the previous one. The phrase “and now” is also a textual marker, but it links different ideas in the same time periods.
Here, the first “and it came to pass” shows Laman’s and Lemuel’s response to Nephi’s lecture. Nephi responds, and the emphasis is on the Spirit protecting Nephi from his brothers.
The next “and it came to pass” shows Nephi’s response to them. The “and now” is part of the same discourse to his brothers, but separates out the example of God’s power to the specific thing that has been requested.