DEAN - research from Australia

[Forfarian]

Considering the Speymouth location, could this Dean line be descendants of the Davidson Clan?

It's not impossible, but at this stage don't waste time over it.

According to The Surnames of Scotland Dean originates in a place name, such as Den in the parish of Kildrummy or Dean in the parish of Ayr.

There is a documented William Dean alias Davidson in a record dated 1703, who was one of several individuals named Dean, otherwise Davidson, who acknowledged themselves to be kinsmen of the lairds of Mackintosh.
 
There is a Clan Davidson web site at https://clandavidson.org.uk/ but I strongly recommend you not to get bogged down with a speculative clan affiliation unless and until you have tracked your own Deans back to a definite connection with it.