I agree, you can buy an ALR up to six weeks in advance
I made my reservations over three days, each taking about 20 minutes, at Glasgow Central
I also made some via email, as seats could not be booked at Glasgow Central
Cross Country, ATW, and WSR all managed to process my requests by email
First ScotRail also made two adjustments and one additional reservation by email
None of them asked for proof of the ticket, I simply advised that I held a First Class All Line Rail Rover valid until 03 July 2009
I also had First ScotRail add a note on my Euston - Fort William that I may be boarding at Crewe instead of Euston