"as" is pronounced 'ah' in front of any word starting with a consonant.
"as" in front of a word starting with a vowel sound will be pronounced 'aZ' to create a smooth liaison with the next word instead of a vowel conflict (aZu instead of a-u)
Sure, but with another color adjective like "vert / verte", (VER vs VER-T)you would hear it, so it is better to learn correct forms even if you cannot hear all letters.
No the U in French never had a Y sound to start with, it's a vowel that English does not have. You pronounce it like the English name of the letter E but with your lips rounded.