Well, more questions than answers but here goes.

Why term it a "joke"? You're not going to get much love on this forum (so to speak) with that terminology or attitude. Advert, as you later say, or challenge or. . . almost anything other than joke/costume/other terms that label the kilt as something other than a natural choice of garment.

No such thing as "correct pleat for dress." Knife-pleat is very traditional but Matt Newsome presents good information on the traditional status of box pleats as well. But knife pleating is probably most recognized as trad Scottish so I'd call that your best bet.

Where did you get "clan colors" for a German surname? As you'll also read here, the concept of clan tartans was pretty much invented during the Victorian Era (with a few notable historical exceptions). It sounds as if you already have the fabric, so go ahead. Make sure he knows the name of the tartan and some background of the clan it's associated with, so he can treat it with respect and answer basic questions.

15, 4, 4, leaves me scratching my head. Thread count? but that's not a tartan count. Pictures would help with suggestions for pleating, but the most direct way is just to pin up some ideas (to sett, to various stripes) and see what works best. Do you have any instructions? There are some available online (but you get what you pay for) and most here recommend Barb T's book The Art of Kiltmaking which is available from her website.

Hope this begins to help and that I haven't completely misunderstood your post!