I confirmed it using the detailed view through HWINFO as well. I haven't had any problems with CAS mixing before. My dad's K95 runs 1 4gb module at 11/1600 and two modules of 4gb at 9/1600. This is completely fine in my experience. I'm not sure how it happens automatically but the system adjusts the CAS settings and operating frequencies in steps - 11/1600 would work as 10/1333 or 9/1066 etc. I assume this is why if you go to manufacturers' websites (like Hynix) they list their modules that they support CAS latencies 5,6,7,8,9,10,11,12,13 etc. even though the modules are specced to CAS 11 at 1600MHz. So, again, I don't know how it works, but as far as I know, frequencies and CAS latencies are changed around between different modules until a match is found.
I don't know what answers specifically you're referring to, aeolisio, but I distinctly remember reading months ago that the internal set of slots is not paired - again, this might be a difference between motherboard revisions.
As I know it, what does stress the memory controller is using 1.5v memory on Haswell CPUs like i7-4710HQ that officially only support 1.35v memory. However, on Ivy Bridge CPUs like i7-3630 using either is 100% fine but I'm not sure on mixing. Aforementioned K95 had it exactly like this - it used to run on one 1.35v and one 1.5v module till i took the 1.35v one and put it in my G751 and instead replaced it with the two 1.5v 9/1600 modules I talked about earlier. Been working 100% stable under extremely heavy leads (video editing/ramdisk) for months. My G751, as expected, does not like 1.5v memory much but it does work fine with it (only tried this a couple of times, I run matched memory atm, essentially same Hynix brand, same latencies, frequencies, voltages etc but with different block structures)
I'm not saying ANY of this is fine - it just is in my experience.
EDIT: Just remembered I've seen some G751s around forums with stock sticks in slot 1, 2 or 4 but as far as I know, the stock 8 gig stick always goes into the same physical place on the mobo so maybe the motherboards between models really are different in layout.