LeDoyen

the top rad is taking the heat of the bottom rad plus the GPU.. it's the only radiator working with 24°C air to cool the water. the top rad is trying to cool water with whatever air temperature is inside the case.
With an aircooled GPU, it's a bit of "choose your poison". You could run both rads as intake and be done with the GPU heating the loop, but the case and the GPU will become hotter.
Both as exhaust may work (with the back fan set as intake), but you dump GPU heat on both rads now. Still it may be better..
 
with everything watercooled, all exhaust with back intake will keep the case cooler, and you can remove the bottom dust filter too since it wouldn't be needed anymore, increasing airflow.

the two prongs on the fan hub should go inside a small molex connector plastic shell. if it's not floating about in the box, it's missing altogether. this connector then goes to one of the LN Pro channels obviously.
For the LL lighting, you have to set it up in the AIO lighting tab on iCUE. Out of the box, only the 3 pre installed fans are set up i believe.

the GPU will always connect to a 8 pin socket on the PSU. the 6 pins ones have all sorts of voltages, and different pinouts.
but if you take a corsair sleeved PCIE cable like those
https://www.corsair.com/us/en/Categories/Products/Accessories-|-Parts/PC-Components/Power-Supplies/premium-individually-sleeved-pcie-cables-with-single-connector-type-4-generation-3-config/p/CP-8920172
.. or a cablemod cable, not a custom one, just a PCIE for corsair type 4 PSU they have in catalog, you can pull the +2 bit all the way back to the PSU, bundle it there and hide it out of sight.

It is a standard power socket, but it was previously only used on big PSUs sold in 120V countries if i remember well, because to get 1500 - 1600W out, they would go over the 15A rating of the C13 plug. In 240V countries, it's not an issue. That plug is usually used up to 20A, that's why we rarely see it.
I guess they just rationalized and went C19 worldwide for the 1500W HXi
bottomline, since you use it on 240V, it's totally suitable.
 

the PCIE cable for GPU has a 8 pin connector on the PSU side, and a split connector for the graphics card that is a 6+2 pins.
Since you have a 6 pin GPU, youi just separate the 2 pins that are not needed and connect the 6 pin. then you can tuck the two unised pins with a zip tie.

You can spin that PSU around, fan up and you'll have more slack for the cables, on top of better PSU thermals. kill two birds with one stone

and if you really need USB3 speeds, you still have that option to get another internal header, and external ports as a bonus : 
https://www.amazon.com/ULANSeN-Express-Expansion-Internal-Connector/dp/B08H1TZPG7/
there's tons of such cards on amazon with different ports options

eww sounds like a messed up bearing...
the fan shouldn't be noticeable like that, maybe a little wind sound but not much more.. I'd open a support ticket with corsair to have that looked into.
Usually for the PSU fan, it starts up if the load is high, and/or if the temperature sensor inside reads high.
Even if the CPU or GPU aren't doing much (powered by 12V) , it's possible to have a high load on another rail, like if you have a lot of USB devices or a lot of RGB (powered by 5v). that can cause the fan to spin up to cool off that part of the PSU.
It's not easy to say what makes it spin, but regardless, it shouldn't make that noise.

you do not need to connect them both.
The 8 pin cable can provide over 300W already and your CPU is FAAAAAR from needing it 🙂 Even a 13900k would not need both cables.
you can leave the 2+2 unpopulated and only connect the 8 pin you have.

for the most part they are the same. the RMi uses type 4 cables, and the HXi uses type 3 cables.
They are all identical except the 24 pin cable going to the motherboard. you'll need to get a type 3 motherboard cable to use the HXi
https://www.corsair.com/us/en/psu-cable-compatibility

here's 19 pages for you to read ^^'
There's unofficial workarounds inside.
 

it is compatible. the PSU uses type 3 cables but the type 4 pinout is identical except for the motherboard conector, which you don't care about 🙂

for RGB you need individual fan control obviously, but for PWM you do not. It will be a lot easier to manage if you use SATA powered PWM hubs and group the fans that will be fine running at the same speed.
If you don't need to have different speeds between front and side fans, you can plug all 6 on a PWM hub and drive that off a single PWM channel.
If you do a push/pull on the AIO, maybe use another PWM hub for the 3 additional fans (one PWM channel more). Then you have one last channel for the back fan.
Then you're left with having to install two lighting node core to manage the remaining fans. These should be in the 3 packs of QL already.
Of course you're down one USB header, so it seems you'll need one of those :
 
It's just one way to do it, there are tons, but as soon as you install a USB2 powered hub, it gets easy.
Usually it's best to use as few commander units as possible and rely on PWM hubs to drive large numbers of fans, as iCUE sometimes struggles when there's many controllers (and they are expensive).
RGB controllers don't pose that problem so, you can connect more of those.

as much as possible, it's better to use discrete cables. two connectors : two cables
 

the waterblock is a tiny bit less efficient in reverse flow, but the performance hit is really small, maybe a degree or so.
It's up to you to see if you want to redo the connexions, but it's not a big deal if you use it in reverse flow.
If you plan to add a radiator and a GPU block in the future, you can wait to get there to correct the flow direction.

the RMX can use any type 3 or type 4 PCIE cable (the PSU is type 4, but both type 3 and 4 PCIE have the same pinout).
So you can get extra cables in the corsair shop, or on amazon, or any retailer that sells those cables.
Apparemment, Amazon FR n'a que les kits de cables premium Corsair qui sont plus chers, mais sur le shop corsair on trouve les cables basiques à l'unité pour pas grand chose 🙂
Cable plat https://www.corsair.com/fr/fr/Catégories/Produits/Accessoires-|-Pièces/Composants-de-PC/Alimentations/Type-3-Flat-Black-Ribbon-Cable-PCIe-6%2B2-pin%2C-single-connector%2C-Compatible-with-all-type-3-pin-out-PSU/p/CP-8920130
Cable tressé https://www.corsair.com/fr/fr/Catégories/Produits/Accessoires-|-Pièces/Composants-de-PC/Alimentations/Type-4-Sleeved-black-PCIe-cable-with-pigtail-connector-and-capacitors-for-Type-4-PSU/p/CP-8920143

For the front panel, have a look at the quick start guide from the product page :
https://www.corsair.com/corsairmedia/sys_master/productcontent/WW_7000D_7000D_Airflow_QSG_AA.pdf
Page 12, you have all the connexions

well in practice you only need the 8 pin one, even with a 13900k.
But all CPU cables should split as 4+4. Make sure you have the right end on the PSU. the one that does not split is on the power supply, the ends that splits goes on the motherboard.

yes they are compatible regardlless of him having an old or newer RM. type 3 and type 4 PCIE cables are identical.

in europe they should all be kettle cables (IEC C13 plugs) i believe.
Otherwise, it's likely to be a C19 plug ?
https://www.amazon.co.uk/iec-c19/s?k=iec+c19

Yep it is.
The cable is labelled as "Type 4" but it is the exact same pinout as Type 3 that your PSU uses.

nah it's actually worse to switch it off physically everyday.
I was worried that's what was done, so it's probably not a damaged primary side, which is usually the easiest to diagnose. From there you really have to followup with the support ticket (patiently ^^') to get the RMA accepted.
Since the problem is not load dependent, it really looks like a dodgy PSU, and not a PC problem.

well, SATA connectors are rated at 4.5A max per rail. If you count ~500mA per fan on full white for maximum current draw on the 5V rail,  that would be a theoretical of 9 fans per unit, if the controller itself can take it.
4 lighting node core seem like a better solution ^^'

SF power supplies are type 4, so it works.
https://www.corsair.com/us/en/psu-cable-compatibility

the EXT_FAN is for an optional fan extension card. it's not a fan header. it's a 5 pin connector too, fans are 3 or 4 pins 🙂
look page 1_18
https://dlcdnets.asus.com/pub/ASUS/mb/LGA1151/PRIME_Z390-A/E15017_PRIME_Z390-A_UM_V2_WEB.pdf
To avoid cable clutter, you can connect the case fans cable to CHA_FAN2 (chassis fan 2) on the edge of the motherboard.
You can then set a fan curve in BIOS, or install Asus AI suite in windows to do the same thing, and set the kind of speed you want.
 
for the front panel USB3 you have two choices mostly :
An adapter cable to connect to a USB2 header, but you'll have slower USB2 speeds : https://www.amazon.com/LINKUP-Motherboard-Compatible-Converter-Chipset/dp/B084D58HBP
A PCIE USB3 card with an internal USB header, and here you'll have the full USB3 speed. That one is just an example, there are tons of models with more external ports etc  : https://www.amazon.com/FebSmart-Ports-PCIE-Self-Powered-Technology-No-FS-HA-Pro/dp/B089ZXXVHY