Dave, I'll give you the benefit of the doubt that you're not a vexatious troll, and respond to your points:
- Distraction during safety briefings is a valid concern. The vast majority of customers, including frequent travellers are unfamiliar with the onboard equipment. Even some aircraft of the same type within an airline have variations that need to be pointed out (i.e. door operation, location of lifejackets). Airlines have a legal obligation to give a briefing to pax, and to do all possible that the passengers listen to and understand the briefing.
- There are also examples of passengers involved in emergency evacuations recording the evacuation (Polish Wheels-Up landing is a recent example available online). Obviously, passengers should be free from all distractions, particularly when involved in an evacuation. A passengers only conscious thought should be getting out.
- Loose items are a real threat, not just in a crash scenario, but also in non-normal situations such as a rejected takeoff. The cabin crew instruct you to store your items in overhead lockers or under the seat in front of you for this reason. Passengers are permitted some objects, such as books, as these tend to be relatively soft and less liable to cause injury. Your phone/camera in its hard plastic case is a different story.
- The threat of interference is a real one, although instances of interference are not likely to be reported in publicly accessible domains. Your point regarding crews recording from the flight deck is moot. Some airlines have differing policies regarding PED use for crew versus passenger use, as some risk is mitigated by having the device under the control of a crew member. Furthermore, whilst there are examples of flight deck videos which were undoubtedly sanctioned by the airline concerned, I'd suspect that many videos were in fact recorded by crews acting outside their company policies and/or illegally.
At the end of the day, passengers (and crew) are required to follow the rules in force at the time. If they do not, they do so at their own jeopardy. I would suggest to you that next time you are on a flight and you feel that there is a "grey area" around what you can or cannot do, you ask a crew member. They will tell you in black and white what is acceptable.