Could I try and help with this - I think also that one of my posts on this forum might be in the back of your mind.
Sarah of course needed a tiara. The Fergusons did not have a family tiara. Garrard, as Crown Jeweller was an obvious source to borrow one. And yes, apparently the Queen was alerted to the position, whereupon she went ahead and purchased it.
I stated on another thread that the reason for this was actually told to me some time ago, although at that time I misunderstood it:-
Traditionally, one would have expected in the normal course of events, that the Queen would have given Sarah a Tiara from her own private collection. But the story that I heard was that the Queen did not want to go down that route 'as she did not want to give Sarah anything that she might not get back'. This seemed a strange story at the time, because it seemed to indicate that the Queen was already having doubts about Sarah's suitability. But in fact, what I had been told I had actually misunderstood. What I was told should have been placed in context, which I did not do. The reason was not so much that the Queen was having doubts about Sarah per se, but that behind the scenes, Diana's marriage was falling apart in 1986 [although of course this was being denied by BP at the time]. Thus, it was not so much that the Queen was having doubts about Sarah back in 1986, but that her advisors were anxious to protect the Queen's property because of what was happening with Diana. At first glance, therefore, it appears that Sarah was being penalised because of Diana, but in fact, a few years down the line, when Sarah's own marriage did fall apart, HM's action in buying a tiara from Garrard seems jolly sensible.