Here is the guide.

Your system is running in CPU Safe mode. This is when SMC believes there is a risk of overheating if allowed to run normally.

I would start off running the onboard diagnostics to see if an error pops up. If it does tell us the full details. Here’s how to get to it Use Apple Diagnostics to test your Mac

To check your battery and charging logic install this gem of an App CoconutBattery take a snapshot of the Apps main window while the charger is plugged in and post it here so we can see it as well.

Sometimes we have to throw in the towel as the costs and risks are too great. Sadly, I think that’s the case here.

A bad display and a deeper logic board problem doesn’t make economical sense to fix.

You clearly need a new display and as the logic board was locked (likely both user account and iCloud), it will require a new board. As the seller failed to clear the user account and break the iCloud ownership so it could be sold as a clean system a new clean logic board is needed to resolve these and that still doesn’t deal with the other damage it has.

I don’t like telling people this and I’m sorry the time and funds you might have lost thinking you could bring this system back alive. Can you get your money back from the seller?

聞いたことのない業者だったので調べてみました。

製品の性質上、修理を請け負う（一般向けの）業者はほぼ存在しないでしょうし、

何らかの形で修理方法が公開される可能性もないと思われます。