Unfortunately you've conflated different issues and come up with a conclusion that doesn't help understanding of the thread topic.
Flying is an efficient way of moving people where the distance is large and there is a need to reach the destination in a reasonable time. The high-bypass jet engine is an efficient way of propelling a vehicle at high speed - if it wasn't then airlines wouldn't use them. Flying in less-dense air at high altitude allows mass-transport of people at speeds of circa 900 km/h. It is unlikely that ground-level wheeled transport will ever be able to achieve those speeds at equivalent energy consumption levels. Flying is also unrestricted by geographic features (mountains, rivers, seas, oceans) that land-based transport requires complex engineering (and energy) to overcome.
Whilst flying can be a useful way of transporting people it's hardly an efficient way of traveling for the vast majority of travel which would be undertaken by those using HS2.
However even where there's an engineering heavy solution (like for instance the Channel Tunnel) these trends to be for fairly short distances and so given that rail passengers have a lower carbon footprint than flying (especially those on electric traction) then the overall impact is still lower.
Anyway there's a LOT of concrete needed to build an airport including the taxiways, runways, aircraft stands, buildings, etc.
In the medium-term eliminating fossil fuels from aviation may worsen energy efficiency as alternative propulsion systems using propellers are inherently less efficient at altitude and speed. So it will probably get more horrendous rather than less, until more efficient propulsion systems are developed.
In the meantime trains will produce less and less emissions as the energy grid gets greener. We do not really have the luxury of of waiting for aircraft to get worse before they get better.
Just to get back to levels of emissions from UK international aviation seen in 1990 we'd need to halve emissions from it. If that's not possible we'd have to cut by 1/4 emissions from road travel just so that those two combined went back to the 1990 level, even if there was no increase in flying and no worsening of emissions.
Demonstrably untrue. A car being driven 20 miles by a worker on their way home at 10pm is more energy efficient than a 10 tonne bus that will need to make a 40 mile round trip. Likewise a train.
If the bus or train is filled with more people then as a mode they become more efficient. But that involves applying constraints that don't exist in the absolute statement you made.
Adopting an integrated transport strategy with energy efficiency and environmental impacts taken into account requires us to accept that in some situations car travel is better than other motorised forms of transport. Train=Good/Car=Bad is too simplistic to have practical application.
Whilst it's true that the individual vehicles in question are more efficient when traveling that is not how you measure efficiency.
Even if it was then it's still not taken into account the emissions from construction of the vehicle and the infrastructure to move those vehicles.
Whilst a fleet of buses will produce more emissions than 5 cars when carrying 5 people, it is rare that is ever the case. As such you need to look at average vehicle loadings to see how is the best way for people to travel around.
On average then a train is better than a bus and a bus is better than a car. Yes you'll be able to find some journeys where the opposite may be true, for instance the train returning to a depot. However those are small inefficiencies are more than offset by the times that there are buses and trains carrying more people than there are seats.
On the plus side, a large fleet of grid-connected EVs provides scope for energy efficiency and grid security improvements with electricity being stored when there is surplus generation and the availability of rapid backup in the event of major generation failure.
Who knows, one day electric cars might help keep the trains running when a wind farm trips out.
A single wind farm is hardly likely to make a dent in the capability of the grid to keep the trains running and the lights on.
Whilst recovering power from charged batteries will help smooth out peaks and troughs in power generation is going to have a limit on what can be recovered.
You'd be better off building more pumped hydro power stations (like Dinorwig) which allows you to pump water to a reservoir when there's excess power being generated and then generate power when there's a shortage of power.
The storage capacity of Dinorwig is 9.1GWh.
Compare this to the storage capacity of a Tesla battery of 100kwh.
That would mean you'd need to take back 5% of a full charge of 1,820,000 cars just to have the same amount of energy.
That's getting in for 7.5% of all cars on the road being able to give back 5% of their charge compared to 1 power station.
If there were 10 such power stations then to be able to draw on that much power would require about 75% of every car on the road to be able to give up 5% of their charge.
Increase it to 20 such power stations and you'd be looking at 75% of cars needing to give up 10% of their charge. Now that's fine if it's overnight, but during the rush hour then very few of those cars will be plugged in.
As do motorcycles, buses, coaches, trams and trains. If you don't believe metal-metal wheels generate particulates then google is your friend.
However nature is more easily able to deal with metal particles than rubber. In fact there's an argument that by putting iron filings into the oceans that it would increase the ability for then to absorb CO2 emissions. (Again Google is your friend).
Now whilst steel isn't iron it is made from it and so, although likely to be less effective than iron the outcome would likely be the same.
However trains are designed to run long distances without breaking and mostly manage to do so, especially compared to cars where they will have to show down or stop for other traffic or due to infrastructure layout quite a lot.
Now whilst the brakes of a train have to do more work than a car, again it comes down to the number of people being carried for the weight. However, probably more importantly it also comes down to how hard the brakes are used. Trains show down much more slowly than cars, mostly because drivers of trains know where they are stopping and so can drive accordingly.
As such the problem of brake dust is a much smaller issue for rail travel than it is for car travel
What you call 'fiddling' I've always known as 'Transport Planning'. Stuff gets measured, other stuff gets estimated, it all goes into a model and out comes an answer based on science and mathematics rather than personal opinions. If you do that kind of thing for a while you start to understand that different solutions are needed for different types of problem. There are few absolutes, and applying policies that are unduly based on generalisations often ends badly.
In response to the assertion "The more people who use public transport the more efficient it becomes" I would say "The more people who rely on public transport the less efficient it becomes". I'd suggest the latter is more relevant to the discussion about HS2.
OK then, using science from a Transport Planner to a Highway Engineer (who has picked up a lot of knowledge of Transport Planning due to working in small companies where the boundaries between the job roles are very fuzzy and so has been tasked with undertaking most tasks on both sides of the fence, in fact struggles to understand where the fence actual is at times), please explain to me how car travel can be a more efficient method of travel at a macro level when there's more reliance on public transport.
I can see how on some journeys car travel could be better, however even then for many of those it would be better to walk or cycle. However, when viewed from the perception of trying to reduce our overall carbon emissions it is generally better for people to use public transport than the private motor car, especially for the 85% of the population who live in urban areas (settlements with a population of over 10,000).