You have done a great job with the RAM and battery test, but I was wondering, have you tried resetting the CMOS battery? If not, please try do so by pulling out the laptop battery and the power cord of the laptop. Open the lid and localize the CMOS battery which lies next to the processor and it looks like a watch battery. If you have another CMOS battery, you can replace it with that. If you don't, leave your current battery aside and wait for some minutes for the electricity to discharge. Then, plug the CMOS battery back in and the power cord. Try to start the laptop again.
Let me know if that works. Cheers.