Yes to all your questions, with
one addition:
I don't have direct experience of using a ticket after its date, but it is written clearly in the FAQs that you can use it for 3 months.
Thanks. Should have been able to look up that myself, since I did try out the link provided above. Sorry for being lazy.
We're not going until April, and I was considering a private car hire to the Hyatt Churchill, but the HEX should be faster and if these codes are still valid the cost might even be lower than the private car. Still toying with the idea of taking the tube to Green Park, cab fare would probably be about the same as from Paddington, but not sure how much that 30-minute difference means to us at the end of an 18+ hour day of travel.