CPU reading 128C forcing fans to always be at full load

Try using hwinfo64 or realtemp. 128c would mean a massive meltdown and the system would crash due to thermal shutdown before then. I'd tend to believe hwmonitor over speedfan, speedfan tends to be glitchy at best and I wouldn't rely on it for any accurate temp readings.

It may not be the components at all, instead it may be a very glitchy program like speedfan which can't make accurate heads or tails of the sensors. Hwmonitor isn't getting false readings and the sensors haven't changed. The only common denominator in the poor temp readings is speedfan. That's why I suggested trying another program like realtemp, it's pretty accurate and a small program. See if realtemp matches what you're getting from hwinfo. If it does, it will confirm that speedfan's temp readings are worthless. The only issue I'm seeing is a poorly coded piece of software, once it's eliminated from the equation there is no issue. It could be that speedfan is internally combining temps from various sensors and adding them together, reading a sensor that doesn't exist so is coming up with some random arbitrary value or any number of possibilities.

The problem is in the software, not the hardware. If I have a broken speedometer that reads 0mph even while I'm driving down the highway, I can keep replacing the motor 2, 3, 4 times and it still won't make the speedometer work properly.

Go to google and use "speedfan reporting wrong cpu temp" as the search term. You'll see this same issue has been going on for years, some even reporting the same exact temp you're getting. It's poor software and likely not worth the space on your hard drive unless it's actually managing to function and control your fans. If it is, then maybe keep it. Otherwise I'd toss it and replace with more reliable software.

Ok so the problem is the fans aren't being controlled properly using the fan hub in the noctis 450. I'm assuming you're using the built in fan hub right? The fan hub should be connected to the power supply for power. It should also have a connector running from the fan hub to the cpu_fan header on the motherboard. The radiator fans should plug into the fan hub on the cooling block of the h100i, then since the 8 fan hub of the noctis is plugged into the cpu_fan header, instead of plugging your h100i in there you should plug it into the white port on the fan hub (using extension cable if necessary to reach it). The rest of the case fans will plug into the various ports on the noctis fan hub on the back.

Doing it this way, all the fans should be pwm controlled from the motherboard via the bios. All fans will run at the same speed as one another though, so if the cpu fans have to speed up all the rest will speed up with it. Hubs tie all fans together and don't let them run at independent speeds. In the bios there should be a setting for cpu fan speed, either a quiet setting, auto setting, manual setting, full speed etc.

I think you have two different issues going on. One, the fans are spinning at full speed either because the hub isn't working properly, isn't connected properly or the bios isn't set to properly control them via pwm. The other issue is the software you're using (speedfan) is reporting a false high temp. Since hwmonitor is reporting a more likely temp of 35c, it's doubtful that high temps are what's causing your fans to spin at full speed.

The high temp reading is faulty on the part of speedfan and has nothing to do with the fans spinning fast unless speedfan is somehow interfering with them and using it's bad temp reading to justify spinning the fans up. Rather it's more likely that hwmonitor is correct, your system is running cool (albeit noisy at the moment) and there's something not quite setup right with the pwm noctis fan hub and/or an improper bios fan setting that's causing the fans to run full speed.

If nothing else, try connecting just the h100i directly to the cpu_fan header and disconnect the hub (let your case fans stop spinning) and see if the motherboard is controlling the h100i properly - allowing it to spin up and spin down variably. If it does, then try connecting it back to the noctis fan hub and make sure the fan hub is connected to cpu_fan, not cpu_opt. Cpu_opt may not have complete pwm support which may explain why the fans are running full speed connected to the hub.