4) Re: Security plan. In fairness, one doesn't need to be a "security expert" to have some sort of security plan. These days, we should all have one. For example: What would you/your household do if someone broke into your home while you were there? What would you do if threatened on the street? These are commonsense precautions, not things that need specific security training (although of course, taking the advice of a genuine professional such as someone in the police, military or other defence field would be ideal).
Bear in mind also that there are a number of JWs who have been trained in security, either because that is or was their secular work, or they are ones whose main responsibility is in leading and teaching brothers to be attendants. I don't know how it used to be years ago, and it may not be the same everywhere, but living in a major city I know that many elders who are captains of attendant teams for conventions, etc, have been on official security courses.
Here in the UK, there is a body called the Security Industry Authority (SIA) which regulates and administers courses for security professionals such as door staff. In some cases, versions of these courses are paid for or run by Bethel, and the brothers trained at those are of course members of regular bodies of elders in their respective congregations, so some degree of trained security knowledge is available at a local level, if not in every congregation, at least in most circuits.
In the case of the JWs and their Hall security plan, I believe this has come about from a combination of three Ps: practicality, preparedness and paranoia. On the practicality and preparedness, there have been some genuine cases of serious security breaches at Halls - such as the shooting in Hamburg in 2023 - and at a lower level, numerous instances of disgruntled exJWs or "unbelieving" family members who have entered Halls and tried to disrupt meetings, occasionally violently. And of course, Kingdom Halls are not the only venues at risk. Experience from the USA especially shows that any religious building, shopping mall, school/college or large building where people gather is a potential target for extremists or nutters.
However, there is also an element of paranoia with the practicality, which stems from the constant teaching that all JWs are under attack from the outside world.
Ultimately, I think all buildings open to the public should have a security plan anyway. The JW one is generally around how to deal with potentially disruptive individuals (spotting warning signs on approach/entry, how to engage with people), possible fire or suspicious packages, evacuation (what routes to take, what triggers/signals will be given to alert others) and practical preventative measures. The latter includes many congregations closing their Hall doors shortly after the meeting time begins and posting attendants at the door (and other strategic locations if necessary, like car parks) to manage who is allowed in afterwards.
8) On attendant training, AFAIK all attendants must be baptised. The training covers the kind of things I mentioned above, and these days includes videos (of course - as with everything in JW-land) on what to look out for and how to engage with attendees to determine if they might be disruptive or not (the principles in these videos apply to local meetings too).
I realise that laws and regulations in other countries differ, so maybe not all attendant training is so rigorous, but I would imagine in the USA and EU countries too, there is probably similar structured training to a higher degree than was done in the past.