What are some important questions to ask former alumni of a PhD program we will enrol in?
You have a good start (how much flexibility is allowed to the students with regard to taking courses, choosing co-advisors, as well as about how the PhD process works, what the most important milestones are and what particular challenges one should be wary of).  Note that how the PhD process works is something you might want to leave out because the department materials should already lay that out, and it might take up a lot of valuable time for her to explain that in your interview.  But do make sure to read all the department materials before you meet with her.

I'll propose some questions to add to your list, but there are probably some things I'm not thinking of, and also you'll want to select a subset of these ideas, since it might be overwhelming to ask all of them.


Write a list of the key members of the cast of characters.  Give her a copy of the list when you meet with her, and take your own notes about what she says.  Ask her which of the names will likely be the most helpful for you over the years, and which ones have pet peeves to watch out for (and what the pet peeves are, of course).  Also ask her what other people might be important to know about, that you left off your list.
Advice for preparing for your qualifying exams.
Advice about course load (two courses vs. three).
Advice about work-life balance.
What do you wish you had known before you started at X University?
Tips for surviving TAing.
How to stay safe while working late and on the weekend on campus.
Suggestions for creating a support network.
What to do between now and then to prepare.
Fun things to do on campus and nearby (this will get your interviewee comfortable, taking a pleasurable walk down memory lane -- so it might be a good one to ask at the one-third point).


The more questions one asks, the more one might convey an attitude of anxiety.  But I personally think it's only sensible to ask questions.  Therefore I suggest that you try to keep the conversation light with plenty of humor, to counteract the possible anxious impression.