16 GB Ram upgrade make a difference?(And few other ram related Qs)

I have a laptop that comes with 2 x 4 GB rams (Hyundai DDR3 PC3-12800 (800MHz)) with a GPU that has 4 GB ram dedicated and 4 GB shared , and i have few questions about :

1 - When I play games I usually do it in full hd and have most settings at high and check with performance tab in task manager only to find it consuming 3 GBs max of dedicated memory and non from shared while consumption of system rams may well be under 80% , while this indicates everything is under control would a 16 GB upgrade make a difference in performance ?

2 - If nor the manufacturer or the internet would say the laptop can operate a 2 x 8 GB rams how can i make sure they would work if the purchase would be an overseas non refundable one , would them be DDR3 PC3-12800 be enough ?

3 - Vendors on the internet claim their rams are DDR3 PC3-12800 (1600MHz) is that because when they run at dual channel the bus speed multiplies x 2 ?

4 - Why is it on CPU-Z it measures NB-Frequency to be 2494 MHz while the internet claims for a DDR3 1600MHz the numbers should be 2200 , are some rams better than others ?

5 - There are some vendors that sell DDR3 PC3-15000 (1866MHz) but I only find them as a 4 GB x 2 , would these work better or at all for my laptop ?

6 - I get to see non ECC rams have 8 small chips on board and some have 4 big ones , are there virtues for which over one another ?

First you have to see how many RAM slots does that laptop has, rare are with more than two.
Your laptop may have GPU integrated in CPU (in that case it uses system memory for graphics) or dedicated GPU (which may use own memory but not sure if it does).
1. A game or program will use as much RAM as it needs to even if you have more.
2. RAM frequency that CPU-Z and similar programs show is half that of real, usable frequency because it's DDR (Double Data Rate) which actually means that it can read and write to it at same time which older RAM couldn't. So your memory is actually running at effective 1600MHz .
I doubt that it uses 8 x 2GB but other way round, 2 sticks of 8GB as that would mean there are 8 memory slots and that you can find only in server grade desktop PCs.
More RAM is always better but that counts only if you habitually run into problems with not enough of free memory. About 2GB of RAM per CPU core is some optimum, over that only if you use some memory heavy programs.
3. Memory running in dual channel doubles memory bandwidth but that doesn't equal twice the speed, not even near. Some programs/games can have some benefit but most don't. Difference may range up to 10% if that.
4. NB frequency varies with CPU and memory divider so it's not always same, you can disregard that one.
5. Higher frequency like that 1866 one is not going to help much and I doubt BIOS would allow it to run at that speed but should run just fine at 1600MHz like old one. Unless you can find 2 sticks of 8GB you'd see no improvement.
6. Doesn't matter how many chips there are as long as other things are compatible.

Check here at Crucial which RAM they recommend for your laptop: Computer Memory | DDR4 DDR RAM Upgrades | Crucial.com