In case anyone else ever tries to do something like this and finds this post, the problem was this:
Somewhere along the line, something required to start up the ROM expected an interactive terminal, which is why it worked over SSH and locally, and even worked with running the PHP script from the terminal, but Nginx/Apache/another web server won't provide any interactive terminal.
It doesn't actually need any interactive terminal, so you can wrap the startup command inside a "script" command like this, to lie to it and trick it into believing it has an interactive terminal
@ramco That's weird, I never saw a TV/Remote who shuts down Kodi or the Raspberry, the power button is never assigned to o CEC button, it should only turn off the TV.
Probably it is your TV brand, never heard about that one. Each TV brand as it's own CEC implementation and name, yours might be one of the weakest.
You said you are running Kodi 18.2, Kodi is currently in version 18.8, update it and check, although I doubt it will fix your problem.
Thanks for the resource, this will definitely be something I'm looking into for another possible project. You are absolutely right about the syncing of two being a problem. As weird as my use case is, I only need one Raspberry (from a kooky angle, the video is being distributed to one/both players). After looking into wedges I think that's the route I'm going to go as I easily have access to USB HID injectors at work/off ebay I can utilize that I didn't realize I could use. Thanks for the link though, I'll definitely be using it.
Please refrain from posting the same topic in multiple categories. It creates forum clutter and makes following the conversation unwieldy for all involved. As this more directly relates to 'Help and Support', I'll lock this one. The other thread is located here for anyone interested.
My best bet is not to launch EmulationStation until you press that button. Now, how you go about it would require some investigation - I suspect if anyone would have done it, you'd have an answer by now.