Removing back plate is the only way to get further with troubleshooting and it’s necessary to get a hint on what’s the problem. I’d suggest to try disconnecting the battery connector from the logic board and then plug in the magsafe, your mac should start on its own if the issue is just battery related. If magsafe still shows no light something is probably shorted or dead in the logic board or the I/O board, that might be replaced as first repair attempt.