To answer your questions:
1. Most games are developed in a 32 bit environment. That limits their available address space to 2gb, and 3gb with some magic. Only a few can use more. 8gb is all you really need unless you are multitasking.
That said, ram is relatively cheap and 16gb lets windows keep more in ram available for instant reuse. Or, cache apps such as Samsung magician can use ram for a ssd cache.
2. Intel ram controller is dual channel. So long as you have an equal capacity installed on each channel, you are fine.
2x4gb on one channel plus 1x8gb on the other would perform the same as 2 x4gb on one channel and 2 x 4gb on the other.
Since the ram controller needs to manage all sticks with the same voltage it is harder to manage 4 sticks than 3.
That is theory anyway.
3. Of possible concern is mismatched ram.
Adding 8gb may be an issue.
am is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
That is why ram vendors will NOT support ram that is not bought in one kit.
Although, I think the problem has lessened with the newer Intel chipsets. Still,
it is safer to get what you need in one kit.
If you want 16gb, the safest way is to buy a 2 x 8gb kit of supported ram and sell your old 8gb kit.