Is there a possibility that you shorted out the board? Spontaneous power issues such as these are often due to a motherboard issue. Especially since you tested the power supply and also tried to SMC reset.
Remember, the littlest thing can cause a short and lead to a dead motherboard. You are going to have to use a process of elimination in order to troubleshoot this issue.
* Check various locations on the motherboard where voltage may be present
* Try booting the laptop without the battery connected
* Check for any damaged traces on the motherboard
* Check for continuity on different connections on the motherboard
* Check for any damage to the power button
I know how important this must be to you especially since you have a customer waiting. So please get back to me when you get a chance so we can discuss this further or if you have any questions about the information I provided.