Mathanxiety - 'how is W a valid pronunciation of a word that begins with D...?'
It isn't! That's the whole point of this thread! We should not assume that all names of letters (i.e. the written word that represents the letter) should necessarily start with, sound like, or even contain that letter. Most do, as it happens, which is where the confusion has set it with aitch.
W is not a valid pronunciation of the word double-yew, and neither do we need/expect it to be, because we all know that the word double-yew starts with a 'deh' sound. (its name stems from its appearance, though it would be more accurate to say double-vee as they do in French.)
P is a letter, Pee is its name, 'peh' is its sound. Likewise D, B, etc.
S, F, M, N, are the letters, Es, Ef, Em, and En (or double consenants, not sure!) are their names, and 'sss', fff, 'mmm', and 'nnn' are the sounds.
H is the letter, aitch is its name, and 'huh' is its sound.
W is the letter, Double-yew is its name and 'weh' is its sound.
Simples!