There are two general approaches to making chicken juicy in the oven. The first is to cook a short time at a high temperature. For example, Barbara Kafka's recipe for roast chicken
calls for cooking the chicken at 500 degrees F for less than an hour.
The second option is to cook at a low temperature for a very long time. This recipe calls for cooking for an hour at 250 degrees F, with a high heat sear at the beginning and end of the time. Even more extreme is this recipe, which cooks at 140 degrees F for 4-6 hours. However, low heat will not give the yummy crisp skin.
Neither of these requires flipping the chicken.
However, if you really want the crispiness of the skin, flipping is the way to go. Two recipes from Cooks Illustrated (one and two) both call for high heat and a couple of flips. (As does Barbara Kafka's recipe for cut-up chicken, which I make all the time. Season the chicken, and put in a 500 degree F oven for 10 minutes, flip, 10 more minutes, flip, and 10 or more minutes or until the skin is crispy.)
If you want to go with the classics, Julia Child's recipe for roast chicken from also calls for turning the chicken onto different sides. She also bastes frequently, although the above recipes don't call for it.
There are several reasons why you should marinate before cooking:
Many marinades contain raw ingredients that should be cooked along with the food being marinated, such as garlic or ginger. In some cases this may actually be a health hazard (raw garlic can harbor botulism), in other cases you'll simply end up with an undesirable pungent flavour.
Many marinades also use some amount of acid, which helps to kick-start the process of breaking down the connective tissue and tenderizing the meat. This is an especially big deal for chicken breast meat and tougher cuts of beef or pork, because most cooking methods (except for slow-cooking) don't do much of this and tend to produce a tougher, chewier end result. Once the meat has already been cooked, using an acid-based marinade won't give you much more tenderization; you need the combination of acid and heat.
Finally, many spices release most of their flavour when heated (cooked). Examples are cinnamon, saffron, star anise, and various types of hot pepper or chili powder. If you marinate after cooking, you won't get much flavour from these.
Of course if you are using a very simple marinade such as soy sauce and are marinating only for flavour purposes, then it might not matter much, but that's the exception rather than the rule. Marinating afterward can certainly improve the taste, but most of the time you'll end up with better results by marinating beforehand.
Best Answer
The flavor doesn't come from one thing, but from the sum of its parts. In the recipe you posted you have many different ingredients, for instance sage, mace, shallots, and brandy. All these add complexity and depth, and combine to make the flavor.
As for what is it that makes you personally like pate there's no possible way to answer that on this forum, you'd have to experiment. What I'd suggest is to make the recipe over a few times but leave one of the ingredients out each time and see which of the ingredients makes the most difference to you. If you do that please post the results, I'd be curious to know.