Hey Rich,
I did everything you mentioned, but the screen was still blanking for me, so I reset the power off in smalltweaks to 120 min and no problem.
I'd suggest trying your settings on your machine with a video longer than the 5 minute maximum time on battery to see if the auto-off happens. See if it the screen stays on with your settings, it wouldn't for me.
I suspect the 3 minute avi you watched wasn't long enough, and even on loop that the looping action might be enough activity for it to prevent the screen blanking.
I watched a 90 minute movie last night in bed and everything worked perfectly with the following settings.
control panel - Display - backlight - all boxes unchecked
smalltweaks - poweroff - automatically turn off checked - set for 125 minutes
TCPMP using Gapi driver for video - settings - no backlight turn off box was checked.
Thanks!
Guyturf