1. Is it possible to apply "speculatively" now for an ESTA, or must you have a definite flight and arrival arranged first.
Yes, it is possible to apply speculatively, and I'd suggest its a good idea to do so. The flight details and address are optional items. Apply as soon as possible, as there is a risk the application could be "travel denied" - having a non-refundable, non-changable ticket and then getting refused electronic permission to travel (thus needing to plan and make a visa visit to the US embassy for a visa) would be a stressful experience!
3. Why is the ESTA a) Valid for 2 years, but then b) You need to visit the website for each arrival. Are these not somehow contradictory.
Maybe, but you don't apply for an ESTA each time you travel - you merely update the existing application to reflect the flight you are taking and the address you are staying at. So whilst there is a risk that a first application could be denied, updates for subsequent journeys (within the two years) are just that - updates that won't result in a denial.
4. If making a second visit how do you cross-reference the original ESTA.
When the ESTA application is approved, you get a reference number. You MUST make a note of this and use it to update the flight/accommodation details for subsequent entries. Note that even though they ask for an e-mail address, they do not e-mail the ESTA reference number to you, so you MUST write it down and keep it (although there is nothing to stop you applying again with the same passport number at a later date).
5. If during the 2 years you change your passport do you need a complete new application ?
Yes, one ESTA application per passport.
6. As I understand it the "72 hours before" is just a recommendation and it will actually normally come back in seconds. So is this something that could end up being done at check-in if needed ? Or for F class passengers only ?
Lufthansa have done just that at some airports in Germany - installed a PC adjacent to check-in/departure gates for ESTA. However of course, as the result of an application to travel could be "travel denied" it would be risky to do the application at the airport just before you [want to] depart.
7. How do you demonstrate to check-in that you have an ESTA ? Do you just give a reference number ? What if you just make one up, or use somebody else's ? Are they checked on-line as part of the check-in process ? Likewise for on-line check-in, how do you demonstrate you have one ? And if you do on-line check-in is it going to be one of those things which, annoyingly, "Fast" Bag Drop ask to see all over again, slowing down the Bag Drop process ?
Dunno because I wasn't asked for the reference at checkin. However, on arrival it was evident that immigration knew I had applied, so I assume its keyed to passport number somehow.
8. Will there still be an I-94W stub, or equivalent, to be handed in on departure from the US, or not ? Are the days of these over ?
Yes, still the green form for now. In theory it goes, eventually!!!
A