It's good that you are talking to attorneys.
You want to find a good one, and this article has some great insights into what to look for, especially when one parent has a PD:
https://bpdfamily.com/message_board/index.php?topic=270440.msg12566140#msg12566140If you haven't already, I recommend reading Bill Eddy's Splitting: Protecting Yourself When Divorcing a BPD/NPD Spouse.
As for questions, it's a good idea to think ahead about your goals -- it sounds like you have a goal of supervised visitation for your S2?
Then ask the L what strategy they would take to help you get to your goal. Tell them this will likely be a high-conflict divorce, and ask if they have any experience with those, and what kinds of outcomes they had representing clients with dangerous spouses.
I would also ask pragmatic questions like:
How long can I expect you take before returning emails or phone calls?
What are some ways I can cut costs -- do you charge for opening email attachments, or charge for mailing me copies? Can I save money by picking these items up and copying them myself?
Do you have associates who will be working on my case, or will you be the sole lawyer representing me?
Do you have experience litigating cases in court?
What kind of documentation do I need to get a good outcome in court?
What strategy do you recommend so I can reach my goal? What tactics? Do you recommend a custody evaluation? What would need to occur for me to get a psychiatric evaluation for my H, and how might that work?
How familiar are you with the judge? What is he or she like? What are some of the things that the judge seems to care about?
What should I do if my H files for custody?
What happens if I move out of the house with my son?
What should I do next time my H is violent with my son or me?
Can I record my H without his consent?
What would likely happen if I called 911?
Have you had a cases like mine before and how did they turn out?
How much is the retainer and how often can I expect a bill itemizing how it is being spent?
Are you either BPD or NPD yourself?

Altho, honestly, sometimes I think the legal profession has more than its fair share of NPDs
Sometimes attorneys will tell you that your goal isn't reasonable, or that your outcome isn't likely. That might be true in the short term -- you might not get full legal and physical custody with supervised visitation right away. But don't let that stop you from focusing on your goal. Often, in our cases, we have to use court to shine light on what's happening, and that can take time. You might want to let the L know that you understand this, and that you want to strategize for both the short and long term, with contingencies for what you will do if the courts compromise at first (which is frustratingly common).
I'm guessing that the DV shelter recommended the PFA and PSA so that you are fully on the offensive, which can be a shocking option for those of us who tend to be conflict avoidant and passive. It might be the best legal tactic, and the hardest emotionally.
You are wise to think carefully about next steps and get a legal strategy in place. It's a good idea to get a safety plan in place, too. I took a year to plan my exit and it paid off. My ex is a former trial attorney and I learned that being informed and staying a few steps ahead can make all the difference. That, and a good L.
Your L works for you -- even if it feels intimidating, and even if you feel completely overwhelmed, remember that you are looking for someone who will advocate for what you know is best.
And we're here to walk with you. This board has lots of collective wisdom and people here understand what you're going through
You're not alone.