Evidence to start the suit is the lack of an apology from the family, plus the fact that the father provided the gun. That indicates a degree of support, and potential culpability, considering the plot was fomented within their home.
And, if true that the parents called police to warn them about their missing son, then we need to know exactly what they knew about their monster-child, and when did they know it. His family shares culpability for his actions.

Launch the lawsuit; secure the evidence. If victim families are reluctant to move forward, then some lawyer/law firm should be reaching out to them with a pro-bono offer.

In addition, lawsuits need to be filed by the victim families against the USSS, and all other parties involved with security for the Trump, Butler rally. Secure evidence. Discovery: Again, law firms should have already reached out to victim families to offer legal council. There is clearly a gross negligence case here. But, money is not the point here- discovery evidence is.