You`ve pretty much done everything so it could be either of the things you suggested. Only one other thing to check - Remove battery and power lead. Press and hold the power button for about 45 seconds which will discharge the capacitors on the charging module. Plug back the power supply and try a boot up. (No need for the battery)
Is the main fan making any movement at all? If that`s packed up the laptop might not boot at all but usually it would perhaps start POST and then stop suddenly as the thermal switch cut in.
It might be the CMOS battery but I doubt it so personally I`d get it checked out at a local laptop repair specialist. Expect a nominal assessment fee but they should have the correct diagnosis equipment.