I completely agree with all of your post, especially the bolded part! Somewhere on one of these threads I listed all the drugs that were on her prescription drug list as shown on the documentary. I don't recall Xanax being on there, and if you don't have a script it's hard to get... unless she was buying from a dealer, which I doubt. Marijuana is one thing and easier to get, but buying controlled pharmaceuticals is another.
FYI... if you still haven't seen the docu it's only on HBO through tomorrow.
Unfortunately, Xanex is not hard to get. You can buy it online and have it shipped right to your home from any number of "pharmacy" sites. I don't know if she had any, but she could have gotten it fairly easily.