I'm confused about how Roger's letter to Bree, which he wrote in 1739, could later be in the hidden compartment of the drawer along with the 3 stamps. Previously we read that on discovering the hidden drawer, Bree & Roger found only the 3 stamps in it. I know Roger went back in time after this, but 1739 had already happened. Another question, how does the desk, which was obviously in the Fraser family, (as it's mentioned when Roger is visiting Brian) get to be in Frank's posession - so that he can hide the letter to Bree in it?
Diana's response to the first question can be read here. Basically she suggests that Roger knew that Brian knew about the secret compartment, so in order for his letter to stay in the desk until Bree might find it two hundred years later, the letter would need to be hidden further inside the desk. Indeed, nobody found the letter until they were taking it apart to put it into storage.
Fortunately Diana has also addressed your second question, though with her usual "answer one question, raise another" response :) Check it out here. Hope that helps!