1) Is the ALR valid from 00h01 on Day 1 (of 7 or14)? and does it cease to be valid at 23h59?
Yes, it is valid from 00:00 on the day of validity
I am lucky in that the Inverness sleeper picks up shortly after midnight at Falkirk, so parked my car there and started my ticket from there, the 00:23 service
As it was a Saturday, I paid for the days parking, returned there that night as there are no Saturday night sleepers
On Sunday I started the next five days from Croy
The car remained there with a change of clothes in the boot, which I swapped on the Wednesday of the first week (this facility was not required on the second week)
The ticket expires at 23:59, however you can complete your journey on the train on which you commenced that journey
Therefore you could commence on the Glasgow - Euston sleeper on the final day, once you arrive at Euston your ticket has ceased
2) If you purchase the ALR at (say London), do you have to finish up in London or anywhere on the network?
No such restriction
I purchase mine at Glasgow Central
My journey commenced at Falkirk Grahamston and ended at Glasgow Queen Street (arriving at 00:27 on day 15)