As someone who travels with a wheelchair bound person, we have found it easier since we gave up with the Passenger Assistance bookings and started just turning up for trains (still giving the ScotRail recommended 15 minutes before departure but)
It's not so bad for us as we normally travel from staffed station (Motherwell, Glasgow Central, Edinburgh Waverley, Dalmuir etc.).
Most of the times we would book in advance, then turn up and find out that station/train staff hadn't been told of the booking, and only the staff at the terminating station had been told, which kind of defeats the point of making the booking in the first place.
If another wheelchair user turns up without a booking and you have booked assistance, you still lose the wheelchair space if they get there first. And in the words of Passenger Assistance: "You would need to wait for the next train". Even if that next train is an hour and a half later.
Or the best one, that happened last Sunday, when Motherwell was closed for resignalling and track maintenance, I phoned in advance (like four days in advance) to arrange transport (wheelchair cant do bus replacement services) and the Passenger Assistance operator booked us a taxi from Motherwell to Bridgeton to connect with the train we needed.
Bridgeton is not wheelchair accessible, if I had not known this my friend would have turned up and found him stuck on the road with no way to get the attention of station staff.
Five phone calls to Passenger Assistance later and they still could not grasp the concept that Bridgeton was not wheelchair accessible.
Ended up just going to Motherwell where the staff there actually knew what they were doing, knew Bridgeton was not accessible and just arranged us a taxi to take us all the way to Dalmuir.
To sum up, if the stations your using are staffed, just turn up. If they arent, book and cross your fingers.