suwanee

In general, as soon as they start signing their names as just first name, you can start addressing them as such. No one will ever complain if you are too formal. To be safe, though, you should use Ms. and not Mrs.

You can see the Council of Graduate Schools resolution at http://www.cgsnet.org/Default.aspx?tabid=201. It may be that Arizona is hoping to notify waitlisted applicants of their status as early as possible, so they want to know whether you will accept their offer. If you have the information you need and can make a decision by then, it will help them to know quickly, but they should not require you to respond by March 15.

Graduate schools/departments should expect that you will want to know about the wait list and how it works, so it is certainly fine to ask. Just keep in mind that the wait list may not be ranked and your chances of admission may depend more on who declines the offer than on how many decline. I also think you should speak with the school that is pressuring you for an answer and ask for an extension. If this is a funded PhD offer, and the school is part of the Council of Graduate Schools, they have agreed to give you until April 15 to give your response. However much they may want an answer sooner, they should not require one before then. You can read more about this resolution, including a list of participating schools at http://www.cgsnet.org/Default.aspx?tabid=201.