So, in your opinion, do you think it's legal/OK/advisable for one with Little heritage (me) to wear that particular tartan?Anybody can design a tartan and submit it - the register's purpose is only make sure that the tartans are unique and to maintain the association between the name and the tartan.
Since the link between clans and tartans is so recent and tenuous, for the most part it's best to think of the clan name as simply the name of the pattern, and not think of the clan as somehow "owning" the pattern. There are tartans that are owned, the Burberry Plaid is one example (though it's a variation on Thompson Camel which is an unrestricted tartan), as are a few clan tartans that were designed as such and registered as "restricted", though in the case of a restricted tartan the restriction is one of convention and not law - the mills agree not to weave that tartan without permission. The Balmoral plaid is currently restricted for use by the royal family, but at one time was an unrestricted plaid so occasionally items show up and are perfectly legal to wear.
Sorry if' you've already answered that; this morning's information is still new to me, so I want to make sure I understand it and "get it right".