Could someone please clarify whether the following things make sense while cooking chicken biryani?
As someone pointed out in your last question about biryani, there is a great deal of variation in how the dish is prepared.  However, I will attempt to address your points based on my experience.


Par-cooking the rice is not for the purpose of flavor absorption, but rather so that the rice is fully cooked after the dish is constructed and (in my case) baked in the oven.  I use water only, and add the rice to boiling water, cooking for about 5 minutes, then draining.
In the procedure I use, the chicken pieces are marinated with the spices and yogurt.  Spices are not introduced anywhere else.  When the dish is constructed, the chicken, marinade and all, is added to the layering in the cooking pot.
The spices, herbs, and alliums in my recipe include, garlic, ginger, ground coriander, cayanne, tumeric, a small amount of garam masala, and salt.   These ingredients are added to the chicken, with about 1/2 cup of yogurt.  There is no saffron, rose water, or milk in my recipe.  The chicken marinates for at least an hour, but can sit longer. In constructing the dish before baking I layer, chicken, rice, cilantro, and onions...ending with rice on top and a drizzle of oil.
Correct, as I stated in your previous question.  The oil carries flavor.  It also produces a crust of cooked rice and chicken at the bottom of the pot, which is delicious.
Oil is used once, to cook the onions (in my recipe).  There is no oil in the chicken marinade.  After the onions are lifted out of the oil, I make sure that about 1/4 cup remains in the pot.  After layering I drizzle on an additional 1/4 cup, which was drained from the onions.
My enameled cast iron pot has a heavy cover.  I place a sheet of foil over the pot, then the cover.  It works just fine without having to make a dough to seal the pot. This is then placed in the oven to cook for about an hour. A pressure cooker is unnecessary.