This idea that there is an authority who can grant or not grant the right to wear a clan tartan really does need knocking on the head.

A Chief can authorise which tartan may bear the name of his clan or indeed reject one. That is the limit of his authority.

A Clan society does not have that authority, it only has the authority to permit tartans that include its own name.

Anything else comes down to notions of etiquette or some idea of correctness which will vary from individual to individual and nobody has the power to force an opposite point of view upon another.

That won't stop some folk trying though.