You asked about the most restrictive tartan that would be the Balmoral Tartan, which is restricted to the British Royal Family, Nobody will sell you the tartan material. Now if you wove your own and sewed it into a kilt I am not sure that anyone would notice nor would the cops arrest you.

Even though Scotland and Germany do protect heraldry (coats of arms and titles) nobody protects tartans.

Because you have Welsh blood, pick a pretty Welsh Tartan. The fact of the matter is that only a few tartans out of thousands can be traced with any certanty to any Clan and few people can trace themselves with any certainty to any Clan. So it is all 18th C. BS anyway.