pronunciation - How should "aargh" really be pronounced?

The common interjections "argh!", "aargh!", "aaargh!", etc., is pronounced, as far as I know, the same as "aaa!" For most of my life, however, I (and I imagine many other second language speakers) pronounced it due to its spelling with an audible 'r' and hard 'g', and still do occasionally. In fact, I think this mispronunciation is so common that it has become an accepted alternative, especially when the word is used in humour (as opposed to genuine fright.) I often hear this version from native English speakers.

Questions:

1. Is "aaa!" really the correct pronunciation?



2. Why exactly is it
   spelled with the "rgh"? What purpose do those letters serve?


3. If
   you agree that the mispronunciation has become an accepted
   alternative, can you think of any other cases where a
   mispronunciation gradually supplants or supplements the original?
   (Especially where this is caused by the eccentricities of
   English spelling and pronunciation.)