It is my understanding that the Q source is thought to be a collection of sayings from Jesus , which follows the structure of the Gospel of Thomas. Is it unreasonable to think that 'The Gospel of Thomas' could possibly be a copy or descendant of an earlier Q source that was translated into Coptic (with or without Gnostic influence)?
It's unlikely.
Q represents the material that Matthew and Luke have in common that is not found in Mark. The name Q stands for Quelle, meaning "source" in German. The standard theory is that Matthew and Luke both drew on another independent document for that material.
But Q is not a real document. It's a hypothetical construct. We have, short of further manuscript discoveries which would be amazing, no way of knowing whether there was a document that corresponds to the material we have. Nor do we know if what we have would form the majority or the minority of such a text.
We do know what's in the Gospel of Thomas. GTom is mostly a colletction of sayings that, on the whole, has a very different tone than the canonical gospels. There is overlap between sayings at a few points, but not many.
So GTom is a known text, and Q is a partly known construct. For them to be the same text they would have to be two non-overlapping portions of a larger text, which frankly is highly improbable.
The Q source need not be a widely distributed document at all, it could simply be that the same person or group of people revised Matthew and Luke into their current forms, and they had certainly teachings they wanted to insert so took the opportunity to put them into both.