were there any failures of 9600M GT and 9400M on the Late 2008 15"/mid 2009 15" MacBook Pros?

I don't recall any posts about these old GPU, but that does not mean they were problem free. As these GPU are now 13 years old, there is no guarantee that they will continue to work or be flameproof.

You cannot upgrade a Late-2008 MacBook Pro beyond macOS El Capitan (10.11.6) which is the minimum operating system support for the current Chrome browser/Brave Browser choice. It does not support Firefox > 94, and the Safari browser is horribly out of date for current website security requirements. You cannot remove mal/adware since Malwarebytes requires macOS 10.12 or later.

Re: controlling fan speeds… you might have a look at Macs Fan Control from CrystaliDEA.

(Tip - never set fan speeds below the default settings)

As for the potential of the laptop to fail... one never knows. Maybe you'll be luckier with this one than with the previous. I suggest you use it to its potential and don't worry. Worry is misuse of the imagination.

Sorry to hear this, my friend. But it was bound to happen. That laptop computer was almost fourteen years old and there's no telling how it was mistreated before you got it.

Thanks for the update. ☮️