Stan: First and foremost there are no "right" or "wrong" answers to your questions. Ultimately, the answers will depend on how you feel about them. Subject to that proviso, my opinions are as follows:

1. Since you already have 4 MacGregor kilts, I would go with a family tartan, especially if there is one you particularly like and regardless of the creation/registration date.

2. I suggest that you acquire a 3-button waistcoat for your PC and a 5-button waistcoat for more informal wear.

3. I favor a 3-button waistcoat that matches my PC.

4. If by "diced" you mean "cut on the bias" ie the bands and stripes of the waistcoat are at a 45 degree angle from those in the kilt, then I heartily endorse that idea. They look very good. I have such waistcoats for three of my "family" kilts.

5. This is your most subjective question. Historical tartans only "trump" newer tartans, if you have reason to think they do. Quite literally, they only trump them in age. But, if wearing the tartan of a famous historical clan is more important to you (for any reason) than wearing a more modern family tartan, then by all means go for it!

Good luck on your decisions.

John