Hi René
I apologize that there was little more any of us could do to help you out. The only thing you can really do is ensure with absolute clarity that you have indeed followed Apple's steps on resetting the SMC. As you may have already noticed, this article http://support.apple.com/kb/HT3964 outlines the steps but they differ depending on your model.
I hope that it is simply a sensor that has gone the wayside and it's not the SMC itself or it could be the MLB that needs replacing.
At one stage you pondered whether you should get a new mac or fix this one. If the cost of repair equates a new logic board, which is rather expensive you may want to consider your options and consider a new mac. It's a decision only you can make.
I really am sorry but hope that an AASP will be able to resolve the issue at a reasonable cost.
On your visit to an AASP, I recommend they document the cosmetic condition of your mac on paper so that you have recourse with them or Apple (as an AASP is an Apple partner) should your mac come back damaged in any way that is cosmetic.
I wish you all the best.
PS remember to backup your data before dropping off your mac at the AASP - just in case.
I thought I would do one more search and came across this article, http://support.apple.com/kb/TS3334. I am not sure if this is your mac though. This might just be of little value now, http://support.apple.com/kb/ht4543 (Learn about the fans in your Mac)
I wanted to make one other point about the AHT. In the past, when macs shipped with their own install DVD, unlike today where it's preinstalled with Lion and no discs are included, the discs would include a version of AHT that was specific to that model. In other words, the discs that shipped with your mac would be the AHT you would need to run. The disc has AHT and its version labelled on it, maybe this is why command-D did not work for you?
These are some articles that are relevant on the matter:
http://support.apple.com/kb/PH4510 OS X Lion: Use Apple Hardware Test
http://support.apple.com/kb/HT1883 Apple Hardware Test: iMac (Mid 2007) and later models
http://support.apple.com/kb/ht1509 Intel-based Macs: Using Apple Hardware Test
http://support.apple.com/kb/TS4154 About the "Apple Hardware Test does not support this machine" message
Take care…