Are aviation accident investigation reports always made public? How is the dissemination of aviation accident investigation reports regulated? Are the reports public by default, or can they be kept secret? Are there any known rules on this? 

If the reports are not public, do the people involved in the accident have a right to access the evidence and/or the reports? Can they find out what the verdict (if there is one) was?

This surely depends very much on the country. Please indicate the regional scope of your answers. (My interest is specific but not limited to France.)
Unless there is some risk of national security to releasing a public report of the accident, the reports are released. You can search the public ones (almost all are public) at the BEA website. The same applies for the other aviation accident and incident reports and you can find the available reports on the websites of the group that has jurisdiction over the accident/incident; NTSB for the US, AAIB for the UK, and many other ones.