There may be an explanation (or several) for it's increase in prevalence.
Questions 2 and 4 may be related.
The development of effective anti-psychotic and other psychotropic drugs to control unacceptable behavior, psychosis, anxiety and depression initiated a paradigm shift in the treatment of mental illness. Prior to 1980 severely and chronically emotionally disturbed people were admitted to either short term or long term psychiatric institutions. Most hospitals had a mental health ward that was secure and locked down. Some were smaller private residential facilities, but the vast majority were large state and/or federally administered facilities.
The institutions were expensive to maintain, and in the new era of 'effective' drugs to control aberrant behavior, it was deemed possible to treat more emotionally disturbed people as outpatients once their acute symptoms were controlled. The number of patients needing to be admitted both for short term or long term hospitalization decreased dramatically and gradually most of these in patient facilities closed or were downsized significantly. The ability to admit patients became more difficult.
The stigma of mental illness remains high. People are fearful of what they do not understand. While it is now better understood what causes emotional disturbance now neuroscience is able to study the metabolic activity of the brain using metabolic scans coupled with MRI imaging.
As a result of increasing violence, and criminals obtaining military-grade weapons police have resorted to increasing their own protective equipment and support equipment. Swat teams have evolved to heavily armored transportation devices coupled with entry battering rams. Police have developed an on, off response at one level. Swat teams are called in regularly when an armed criminal event takes place. It often occurs for a 'domestic violence' event. Common sense has devolved into brute force for relatively minor events. This markedly elevates the 'threat' component of an event.
Gun violence under the microscope
Recently a national reaction to episodes of police violence a movement has called police reform mandatory to detach racial profiling from enforcement. Several propositions proposed to 'defund the police' include disbanding present police departments and replace them with new entities. Others suggest integrating more mental health diagnosis and treatment integrated into the police force. Police have recently become much more 'enforcement' entities over 'protect and serve'. The gradual shift has taken place over several decades. It is apparent the current situation is non-sustainable.
Certainly swat teams are indicated for mass shootings at public or school events. The events initiate a heightened sense of awareness and anxiety.
Why is this appearing on a health blog? Statistics reveal this is a public health problem, just as Covid-19 has become. Public health experts who study the situation can make recommendations, but will officials and/or authorities follow their guidance. If Covid-19 sets precedence, it is doubtful. Economic concerns and 'reality' often override science as we observe the current Covid-19 return to normal.