Yes, it's a good practice. The questions don't vary much from year to year, and it will help you begin to reflect on key themes and improve the questions you ask in networking chats, which can help you address key themes in your essays (e.g. skills, your "why", etc)

The process of preparing answers is iterative, and it's advisable to allow enough time. As you network, you may adjust the content of your essays.

Alumni and current students should be the main target of your networking.

Some universities have admissions teams that are more accessible than others. Overall, they may all answer specific questions, but they won't necessarily accept networking calls. Universities like Duke and Columbia have open agendas for you to schedule a meeting with them. MIT doesn´t offer that option.

The recommendation is to do all your research and networking beforehand so that when you interact with them as part of the process (e.g., update emails, etc.), you already have detailed knowledge of the process and your own motivations. For those like MIT, if you manage to establish a close relationship with the recruitment team and feel the situation warrants it, you can propose to have a quick call with them.

No.

You should network to the point where you know the application process by heart, you have strong arguments for your why (Why this Program, Why this University, Why now), and you know alumni who are willing to send referrals. You may also want to ask alumni or current students to help you review your essays.

Some people may need to network with 10, and others may need 50. A sign that can help you know when it is enough is that most of the answers you get are not new to you.

Your recommender doesn´t need to have studied in the university you are applying to. If they have, that is a plus. But it´s more important that they have worked with you closely and can make a strong recommendation with a focus on the five key pillars of an application (read more in: Postgraduate > Step-by-step guidance).