1. Am I correct that my university is in no way involved at this point?
Yes, unless you can convince them to become involved. That may or may not be beneficial.
2. Do I have any duty to attempt to help?
If so, is there a way to do so that does not engage the individual directly (which I hesitate to do)?
Asking the police to do a wellness check may be such an option, but be cautious about records and rights. For example, you will want to look into public records laws. While these help with govt transparency, they also can make the simple act of asking for a wellness check itself a request-able record. This may be compounded if you are an employee of a state school. If, for example, you were at a state school in Florida you would find that any correspondence with your university, with police, or related to the original issue becomes public record, request-able for $12 and your name. You may then be exposed to further, more directed, harassment, or in some jurisdictions can be targeted for harassment charges yourself. Would you like to defend that the contents of your letter warranted such a request to the police, in court, on the record, for local newspapers? You should talk to an attorney about this. Do not trust the police to know the law.
3. Beyond a police restraining order, is there any action I might take to insulate myself and my significant other from any negative actions this individual might take?
Have a secure home and a plan. Talk to your family. Know your options and available actions in various scenarios. Only buy a weapon if you have a clear understanding of how and when to use it.
Buy good understanding of the law. A few hundred dollars to a lawyer specializing in harassment cases in your local area would be a wise investment. Do not rely on your friends, the police, or internet strangers to understand a situation that is rooted in a local legal and societal situation, as well as a set of circumstances you are clearly not comfortable posting in full on the internet. Nor should you be comfortable with that.
Also work to understand your university. HR, campus police, the dean, etc. are not your friends, necessarily. They exist to protect the university, not you. Be very cautious with the prevailing wisdom here: your university is likely to take actions that protect you only when they also protect the administrator in question, student body, etc. Do you have a strong local union? Talk to them. Can you find colleges who have faced similar situations? How did they fare? Tread lightly. In some scenarios you could tank your career even if the person never contacts you again.
Learn by talking to others. You sound like an individual who has lived a life with very little harassment. Frankly, this entire thread sounds like advice from a bunch of white men to one another: the blind leading the blind. Talk to the racial and gender minorities in your life, if you have some who are willing to speak directly with you. They will have dealt with harassment more often, and can tell you about your local environment. Also talk with any friends you may have on the local police force, away from their official capacity. In most jurisdictions, the police cannot protect you well from harassment. Even in the best of situations they arrive slowly. Reports and restraining orders come with an implicit understanding to the individual you wish to avoid that 'this person tried to hurt me'. That can have consequences.
In sum, much of the advice here seems, to me, deeply flawed and divorced from reality. You need to think hard about this. Advice about criminal harassment from an internet forum for professors and grad students, a largely privileged population, well, it's certainly suspect. Find better advice. Challenge your assumptions. This sounds like a scary situation for you. Be smart. Be safe.