With your TrueSmart back cover off but the battery still installed, can you still see where the power button is pushing against the power switch on the main board ? If so, you can push the power button and see if it hits the switch to depress it. Maybe you could also carefully slip a tool in sideways between the button and switch and attempt to depress the switch. I suggest you try with something non-conducting first, if you can reach it with the battery still installed at all.
This is just to check whether the power switch is being depressed far enough to activate the watch. Sometimes the switch has been found to not have been properly soldered to the main board or to be otherwise defective.
Make sure it still won't power on after a battery pull first, of course. I'm sure you have already tried that, but it bears repeating.
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