A> The ram slots are color coded, simply fill the slots that are the same color and you will be fine.
B> As long as they are the same timings and sizes, yes any order is basically fine.
C> In the case of kitted RAM you want the two that are a pair in the same color slots, and yes two kits will work together.
D> For gaming? No, 32Gb is overkill, 16 is more than sufficient. If you have some leftover in your budget consider upgrades in this order: GPU, SSD, everything else.
i think his gpu is enough a 780ti is the best. I doesnt botteneck a good cpu like a gtx titan z. but i know you mean upgrading.