Have you never flown with budget airlines and seen ths distances that they consider reasonable to walk at airports?
Or travelled in First Class on the Highland Chieftain....
As far as an Old Oak Common-Stratford link goes - the journey times would probably make Leeds and Manchester to Paris viable. But with HS2 planned to be operating at maximum capacity as soon as Phase 2 is complete, at least on current plans, there aren't any paths for such services. I'd sooner have that than a Heathrow branch - changing at Old Oak Common for the airport is likely to be less of an imposition than having to change stations mid-journey in London.
(The possible folly of massively long trains aside, but that can be mitigated by things you can't do with an airport, like board from above / below, all along the train, multiple platform entrances etc)
Travelators down the middle of long platforms would seem like a good idea to me. Totally proven technology from airports, too.
Realistically, £56 billion seems about right, perhaps a high-end figure but reasonable, after allowing for inflation. Higher figures, where not scaremongering, are most likely the result of risk management. The government expects HS2 Limited to shoulder all the costs of 'what if it goes wrong', and HS2 Limited are pushing those costs on to their subcontractors. The risk budget can add up to a lot of the project's up-front costs, since the contractors will
always pad their estimates in a way that ensures they make a profit.
If the government is willing to take the risk burden, they can cut out all that padding and the associated management fees. They'll have more uncertainty, but wind up paying less unless the project turns out to be a colossal disaster. And if
that happens, the contractors will turn around and ask for more money anyway.