Sorry there just isn’t any air flow diagrams for the internals. Apple does not offer them and I don’t know of anyone who has made the effort to properly test and create any drawings.
Even still we can use some common sense! The air enters from the center rear grill area going across the logic board and then sucked out by the fans from both bottom and top sides of the logic board
Sorry to say there really isn’t anything you can do. The issue you are facing is within the chips design. NVIDEA used a flip chip design which at the time was a very new method! The solder they used as well as some mishandling of the carrier part. NVIDIA used a high Tin content solder which tends to acquire tin whiskers which shorts connections internally of the chip as well as fractured solder joints. To add to that the logic board to chip solder also tended to fail as well.
What you run on the system is more of a factor! the least amount of graphical work will help! As you don’t want to push the GPU chip running anything heavy for graphics like no videos or games! So writing is about it, sorry.
If you can I would return it ASAP to get your money back!
The last issue you face is the age of this system can put you at risk if you get on the internet. Apple has long since stopped maintaining the Older OS-8, OS-9 or even the early OS-X releases this system used. Assume anything on your system is exposed! Pictures, email or social network and bank account info! You also run the risk of being infected by a virus or malware.