You're placing a PCB on a carpeted surface ... that's not very wise from an ESD standpoint .
How did you remove the bottom aluminum part of the CPU Tray without removing both CPU heatsinks ? Or did it come from the seller that way ?
For releasing the captive CPU Heatsink fasteners from the Tray , you should use a 3mm T-handle hex driver with a long blade . I use a Bondhus model 15356 . It has a nine inch blade . You can manage a lot of torque with one of these , so that's probably what you need with that stuck fastener .
A small tight beam flashlight will assist in verifying the tip of the driver has locked in place onto the top of the fastener if things have gotten a little worn over the years - you simply point the beam down into the hole while aiming with the driver .
It's very rare to strip one of these fasteners ...
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