...it does add intimacy to Darcy and Lizzy's relationship which evolves into romance. But, yes, Mrs Bennet would never hesitate to open and read her daughters' mail. But don't forget, Lizzy is well aware of her mother's habits. I don't know when or if she destroyed the letter, but is this hope? We know she reads and re-reads the letter until she literally has it by heart. In P&P 2 when she's home and telling Jane about Wickham's real charecter, I'm sure she showed no letter to Jane. This may indicate it was already destroyed. :)