No it isn't. It's debatable if the safety features are still necessary with modern wiring and electric code imporovments, but the features are objectively there, and they objectively make the plugs safer.
And the design of these features wasn't because of "substandard" wiring. It is because the UK used to use ring circuits in old houses, which are unsuitable to be protected by central breaker boards with breakers for each room, necessitating fuses in the plugs. That doesn't make the system any less safe. As long as a fuse is present, and the circuits are adequately sized, where precisely on the circuit a fuse is located is irrelevant.
Also, the fuse inside the plug provides an utterly unique advantage that no other country has: The fuse can be used to protect the external wire from over current. Centralised fuses are exclusively designed to prevent over current on the main, internal circuit, they don't give a crap what happens on the other side of an outlet. A central fuse protecting a 16A circuit will do nothing to stop you from pulling 15Amps through a 3 amp cable. A fuse inside the plug, appropriately sized for those 3 Amps, will in fact protect the cable itself, allowing you to safely use low diameter cables on low power appliances. This is particularly useful for extension cords. Other countries without fused plugs need to either just flat out mandate ALL extension and multiplug cords be capable of safely handling the maximum current of a household circuit (e.g. Germany) OR just ignore that rather major safety hazard entirely and just kinda hope that nothing bad happens (e.g. USA) (if you've ever wondered, that's specifically why chaining extension cords together in the US is considered dangerous)
No it isn't. Not in large airplanes. The typical takeoff weight of an A320 is in the neighborhood of 50-70 tons. The pilots do not give a shit if a few passengers weigh a couple 100kg more than the average, and that's a narrow body. A couple PAX being grosly obese on a widebody, with typical takeoff weights in excess of 100 tons, is even more negligible.
Fuel calculations and weight&balance is calculated based on assumed average weights for men, women and children, generally something in the neighborhood of 85kg for men, 75 for women and 30-40 for children (includes assumed average hand luggage weight as well)