Queen Mother
ENVELOPE: "Her Gracious Majesty Queen Elizabeth The Queen Mother," for formal and state papers, otherwise "Her Majesty Queen Elizabeth The Queen Mother,". Less formally "Your Majesty"
OPEN: "Madam" or "May it please your Majesty"
CLOSE: "I have the honour to remain Madam, Your Majesty's most humble and obedient servant"

Viscount
ENVELOPE: "The Right Hon the Viscount ......", or socially "The Viscount ......"
OPEN: "My Lord" or socially, "Dear Lord"
CLOSE: "Yours faithfully" or "Yours truly"

Viscountess (if the wife if a Viscount)
ENVELOPE: "The Right Hon the Lady ...." or socially "The Lady ...."
OPEN: "Madam" or socially "Dear Lady ..."
CLOSE: "Yours faithfully" or "Yours truly"

Viscountess (if Viscountess in her own right)
ENVELOPE: The Right Hon the Viscountess .....", or socially, "The Viscountess."
OPEN: "Madam" or socially "Dear Lady ..."
CLOSE: "Yours faithfully" or "Yours truly"