I've checked this with my Dad because he's the 80s expert rather than me.
He's not sure on the servicing arrangements at Queen Street as late as 1987/8, but he did say that the Glasgow-Aberdeen push-pulls started as a single mk3 diagram as early as February 1981, initially with a mk1 catering vehicle in the makeup, and from 1984 onwards the sleeper returned south within the first return leg of the diagram from, in other words the 11am ish departure from Aberdeen.
He says that the 1525 ex Queen Street (and the 1900ish back from Aberdeen) was the same mk3 set in the early years, but he's less clear what happened later when they had more 47/7s and DBSOs available and they ran around with mk2s on most of the services south from Aberdeen. So it could be that the 90 minute turnaround you mentioned involved two different sets by the time the sleeper was included, and even if it was the same set in the early days, unlikely that it would have occupied P7 at Queen Street for all that time between services.
He's got loads of photos of Scotrail in the 80s which we keep talking about scanning, and some definitely include the sleeper in the 11am departure from Aberdeen. I'll see if he can dig them out and I'll put up anything that's relevant.