Maybe I'm not understanding the purpose of your original post. I assume you've tried changing the settings in Preferences (under Battery, formerly called Energy Saver) for how long to keep the display on when on battery and plugged in? By default it's set to a few minutes, but you have the option to set it to hours or never.
I find Amphetamine useful for times I want to temporarily override my preferences setting without actually changing the preferences settings. This is usually for processes that take a while and I'm stepping away from the Mac- like transcoding, code testing, large data transfers, or long downloads. I find that often if I don't keep the system awake with Amphetamine, when sleep kicks in it interrupts or fails whatever process(es) I was running.