But would a tartan that was created in "authentic colours", yet not containing a sett that has been proven to have existed at that time not be a "viable" scheme?

Viable,definition #3:
Capable of success or continuing effectiveness; practicable: a viable plan; a viable national economy. See Synonyms at possible.

See where my though process is going on this one? If the sett has not been proven to exist in the early 1700s, yet the colour style has been, then would it not be historically possible?

Well, that's my defense for it anyhow. Valid in their context is another way to put it, yes.