Can an alto sax reach low A? Also, I have seen soprano saxes with high G keys... Do alto saxes have this key, too???

I have never seen an Alto with a low "A" key, but in the question of a high "G", there is no required key(except the high "F#" key) its just a matter of fingering the right fingering and your embouchere, there is a fingering and at the moment I don`t remember it. But I will reply once again, later.

to get low A on your alto, play low Bb, drop your jaw and loosen your embouchure, and cover the bell with your knee (sit down obviously!). Not hugely reliable, but fun...we got a bari down to a G!