If you notice there are two transceiver chips together, mine are ELMOS near the shielding can, there is also another RS232 chip, the one on the bottom is there solely due to space restrictions I expect. Everything runs back through the board to the top side. So everything appears to be doubled up, I haven't yet traced the second set yet. I will get around to it soon.
The bus comes in at 12V, gets reduced down to a sane level, appears to go through a hex buffer chip then up to the RS232 chip. From here I presume, but haven't checked, it goes in to the shielded area through another chip and hopefully ends up in the main chip for instructions.
Pressing eject does nothing extra for me, just ejects the disc, I already have a solid 12V input. It sounds as though you aren't getting the battery 12V through to the correct place and it is having to go through the button
I only followed the input voltage section for a brief time, I soon discovered that pin 3 enabled the drive.