I think your suspicion about the Tilby name possibly being a surname at one point is probably correct. I have rarely seen it used as a given name.
It would seem that if it was a surname belonging to a family member at one time, the person must have been well though of. Either that, or it's simply been passed down through generations.
I think a starting point would be to identify the names of the parents of Benjamin Barnes and Jane Goldsworthy. It is possible that one of their mothers was a Tilby prior to marriage. Do you know where Benjamin or Jane were born or anything about their parents at all?
The name Benjamin, it would seem,is coming from the Barnes side. But the name Herbert could be from the Goldsworthy side. There are a number of Herbert Tilbys, some with Australian connections.