Heavy fan usage, low temperature, and kernel_task kicks in for no reason.

First of all, I'm not a native speaker of English, please excuse my mistakes.

As the topic said, my Macbook pro 13-inch 2018 with 4 thunderbolts is under what looks to be a maximum fan speed. The problem shows up even before MacOS (Big Sur 11.1) boots up. Here's what happened before it occurs:

• I was using MBP as normal and noticed a very high temperature than usual.
• I then noticed that (at least) a fan was not spinning at all, so I immediately force shutdown the computer, without looking at the temperature monitor. But right from my instinct, it was probably at least 90°C.
• I waited for it to cool down, opened the back cover, removed the logic board, and noticed that one of the fan (the left one when view from the back) was loosely plugged in. I don't know what caused this, but I proceeded to plug it in properly and reassemble the computer.
• I boot up the computer as normal, but it took a very lomg time to boot and a performance drop was very noticable.

I immediately know that kernal_task is doing its stuff, which causes my computer to slow down to help with the previous overheating issue. This turns out to be true according to ActivityMonitor. After some further inspection and 2 nights of trying out some potential solutions on the internet, here's the situation as of writing this:

• Everything seems fine, MBP is able to run on battery, it charges properly, it is just the kernel_task governing about 70% of the CPU and slowing down all user processes (see picture.)
• It seems like key combinations you press when booting up is working. I think I have reset the SMC and NVRAM as some guides said to, the problem persists.
• I was about to perform the deletion of kernel extension IOPlatformPluginFamily.kext/the plist file inside it (which I know nothing about, just following the guides), however one major problem stood out: my Macbook can not boot in recovery, neither from internet nor from disk. I tried booting in recovery with terminal commands, does not work either. I even allow it one hour after the key combination, it still stucks on the Apple logo with no loading animation.
• According to iStatus Menus, every temperature reading is no larger than 40°C, with the highest being the CPU at high thirties.
• I also notice that the system works perfectly fine without slowing down for about 10 seconds after waking up from sleep or screen timeout. System's CPU usage is about 10-20% for that period.

The problem might be that the resistors malfunctioned when my laptop was overheating, or it could be SMC failing to recognise that the temperature has already cooled down. Either way, I have important files backed up prior to this and am willing to manipulate system files, reinstall the OS, or replace the component(s) if needed.

It is also worth mentioning that I there's a problem with the same fan that was not properly plugged in: it seems that the fan can woggle by a little bit (the other one can't) which cause periodic noise from physical contacts of some sort. But this is probably unrelated, as the laptop worked fine with the fan issue before.

Also, I cleaned everything when I opened the back. This is not the dust problem.

So, if anybody has a potential solution, please share it with me and everyone else that's having similar problems. If you need information, feel free to ask. Also, thanks for all replies in advance.