I normally don't feel a need to respond to other's posts, but, with all due respect, I have to question what I see as a misperception of things on your part.
I have run more than 6 of these MacBooks, model A1342, with 8GB ram and NONE have run "hot" at all, and all but one have had the cracking near the hinge points.
In my opinion, the cracks are related to physical stress, NOT heat.
You may very well have a MacBook that is overheating for other reasons than the RAM upgrade, and also has the cracks near the hinges, but I feel they are unrelated.
On the overheating issue, I would check that the fan and heatsink "radiator" are clean (blow the dust out with canned air). If that doesn't solve the heat problem, I would go in and remove the heatsink, and clean off the old thermal compound, and reapply GOOD QUALITY thermal compound and reinstall the heatsink.
If you wish, and are suspicious of the RAM upgrade, you could first try checking to see if removing the 8GB RAM upgrade and replacing it with the stock RAM sticks solves the overheat problem, but I doubt that it will.
As for the cracks - welcome to the club - nearly all these A1342 plastic unibody MacBooks have them.