Thats the million dollar question. No answer is ever going to satisfy all sides and even if one side got their way, with either no guns ever or no gun restriction ever, you will never be able to stop domestic violence murders. At most you could do is put small dents in the numbers. You would probably reduce the domestic violence numbers more by addressing alcohol use more than any gun restrictions. No one ever wants to talk about restricting alcohol though. Maybe anger management counseling. Cops can show up to a DV type incident with all sorts of options from suggesting one party leave, making one party leave, arresting one (or both) parties, but counseling is not in their normal tool bag. Not unless an officer happens to be quite good at counseling people.
A lot of times TRO applications make very serious claims about historical instances but the applicant decides to go the TRO route instead of pressing criminal charges. The TRO might mention rape, attempted murder, threats, etc. but without a criminal case being made the accused wont be charged criminally.
Going for a TRO or criminal charges present their own problems though and it is impossible to tell which one, if any, will produce the better results.
If I understand the reports correctly the guy had very little on his record to justify any sort of detention. The one thing I would be interested in pursuing is, as you said, a mandatory mental health evaluation. If a TRO applicant is reporting all sorts of mental health issue type behaviors and all they do is say don't talk to the applicant or own guns then there is a missed opportunity. There are going to be some real civil rights issues though given that forcing someone into a mental health hospital is taking away their rights.
I don't think you understand our justice system very well.
Even if the wife filed a criminal complaint against her husband, due process requires an arraignment, specified charges based on sufficient probable cause and evidence, and more than likely he'll be released with or without bail to await trial for
who knows how long?In the meantime, a TRO is the only immediate protection (and I say that knowing it's not really protection) that's available from the system.
So, even if they arrested him and tried to hold him, the system crawls at a snail's pace. The system also must be concerned with the rights of the accused -- including the right to be free on bail if the court decides he needs to maintain his employment, etc.
Often the court (judge) will issue the TRO as an immediate remedy until the trial (it's called Temporary Restraining Order for a reason). If, after the case is tried, the court decides he's still a threat, they can issue a Permanent Restraining Order. From my experience and readings, restraining orders are civil decrees from the court even though they are often issued as a result of criminal behavior. It's only when an RO is violated that it becomes a criminal issue (
criminal contempt of court -- ask me how i know

).
Restraining orders are nothing more than
trip wires that signal to the police they can now arrest him if/when he violates it. It's like a condition of release -- stay away from her, or we will lock you up.
Basically they are giving him the opportunity to continue his attack/harassment with the hope that the threat of being jailed for violating the TRO is enough to stop him.