1. "Best" is pretty hard to define
A couple of strategies you can try: a) Give your pretender magic that your race doesn't otherwise have. Suppose you are playing Man - you might design a pretender with fire magic, so you can find fire sites, craft fire-based magic items and cast nasty fire evocations. b) Pick out a specific spell from the spell manual that you think would be fun to cast, and design your pretender to be able to cast it.
Magic Duel is pretty simple really - if an astral mage casts Magic Duel and the enemy has an astral mage present, they will "duel" and one of them will die. The greater astral skill your mage has, the greater the chance that he will win the duel. The relevance to Pretender design is this: suppose you decide to invest in a magic-heavy pretender (rather that putting the design points into dominion scales or castle), and you include a point or two of astral skill. Then an opponent who has a mage with 3 or 4 points of astral skill can duel your pretender, and all those design points are wasted. So the common wisdom seems to be that you either take no astral on your pretender, or you take a lot so that you will tend to win duels. A third way (as I belive Pocus pointed out) is to take moderate astral, but plan on keeping your pretender out of combat.
2. That's largely a matter of style. I personally tend to favor quality over quantity, but both work
3. If you're going to take 6+ levels in a school, be very sure that your strategy requires it - very few spells need 6 or more levels in a school, so unless you think those spells are very important, taking that many levels is a waste.
4. The stronger your dominion, the easier it is to spread it out into the world. Your troops get a morale bonus when fighting in your dominion.
5. The short answer - supply "radiates" from castles out to 4 provinces, IIRC. The further from a castle, the less supply. For the long answer, read the manual
Build castles where there is something important to protect, or where you need more supply, or where you want to build more troops than the current resouces of the province allows.
6. Build a castle there. The administration value of a castle draws that % of resources from neighboring provinces that you control. There is also an Earth ritual called Riches from Beneath that will raise the resources of provinces under your dominion.