At the risk of repeating myself ... none of those things will happen, because the owners will over sell slots on it - just as they have on the current pair of strips
No they won't. The government have capped the 3rd runway at half its allowed capacity. This means the max capacity LHR will be legally allowed is 83% - so it will massively reduce holding times both airborne and on the ground.
Passengers prefer to use KLM/AFA etc. It is less hassle, and usually also cheaper.
How on earth you can say connecting at CDG is less hassle than LHR T5, I have no idea.
Taking this a step further then is the cost of the projected expansion expected to be funded from revenue generated by greater volumes of transiting pax, the majority of whom originate from overseas? Because if it is then is there not a more effective way of investing the £billions it will take to construct a third runway. All this LHR being unable to keep up with AMS, MAD, CDG or wherever. Then does it really matter, future UK needs would be well catered for by the existing facilities (Subject to demolition & rebuild of T1/T2) Capacity being made available by transiting pax going elsewhere, who really cares ?
Take a look at all major cities in the world and note that they all have a hub airport and hub airline with a significant proportion of connecting passengers. There's a good reason for that. If you reduce LHR's status to an O&D airport, vast numbers of direct served destinations will be lost because they cannot be supported as O&D - and that will directly affect London as a city and place to do business. If people can't get to it easily, they'll go elsewhere.
The argument that "a 3rd runway will only benefit transfer passengers" is fallacious anyway. There are masses of destinations BA would love to serve from LHR if only they had the slots to. It will also as said above decrease enormously operational delays, nice new terminals are good for the passenger experience but ground delays are still a huge problem with runways operating at 99.8% capacity.