The Coppola "diamond series" wines are well-made and usually decent, a cut above what used to be called "sound commercial wine" in the trade. I agree that they seem to be overly oaky, but I'm not sure why since they are usually bought-in wines. Over-oaky can come from new barrels, and, especially from American oak rather than Nevers or Limousine oak. I can't imagine Coppola putting bought-in wines in his expensive French oak barrels, unless he uses the wines to season the barrels before he puts serious wine in them. I'll have to ask my cousin who has a bottling operation and has bottled for Coppola. Back around '96 I actually watched some of that wine being bottled.
I have enjoyed the Claret in a couple of vintages. I wouldn't know about the Merlot. IIRC, I was not thrilled with the Chardonnay, the oak reminded me of cardboard. Often they are not made from grapes on the estate, or even with his own wine (when it says "vinted and bottled by" you know the named producer did not make the wine, he bought it and finished it, perhaps blending it.
"vinted and bottled by"
I understand that the amount could as much as 25% not from the vinter. Sad!
European standards are much higher in this regard.
I would be interested in hearing info from your cousin ;)