Good bunch of questions you have put to the rabble. It is my belief that for a tartan to be official, it has to be recognised by the institution it purports to represent. A clan tartan has to be sanctioned by the chieftan, a corporate tartan by the owner of the business/board of directors, a family tartan by the head of the family
I, for example, could design and have made up a tartan I claim to be a MacLaren tartan, I could register that tartan, but unless it was recognised by the Chief of the Maclaren Clan, it would not be an official MacLaren tartan.

As for restricted tartans, most weavers will not weave it unless you have written permission from the person listed in the restriction (usually found in the documents when the tartan is registered) Some restricted tartans will never be publicly available eg Balmoral, while others may be released for payment of a fee eg isle of skye. Other corporate tartans are only available to employees or as gifts to clients.

Those are my thoughts and I'm sticken with them