It appears the 2.25us period ripple/burst is not present on the mains side, but the differential measurement was coarse and noisy so it was hard to tell definitively. I also found out that when the backlight IC is not powered on (3.3v power on signal is low), the ripple goes away and the 5V and 12V rail are very smooth. I attempted to see if the ripple would appear if I put a load on 5V rail, when I tried a couple hundred milliamps none appeared. When I tried this with the 12V rail a small amount appeared, but not at the same period as the ripple appearing when the backlight was on. I then substituted in a ripple free 3.3V signal for the power on to see if the ripple on that was causing the issue, and ruled that out, ripple still occurs.
I found out that the IC in the scorched looking area referenced earlier in this thread is almost certainly a 5V to 12V boost converter. So, it appears the problem might be in either this boost converter or in the backlight IC itself. As is par for the course on this powerboard, I could find no datasheet for that boost converter IC. I will see if I can make and post a schematic of it, perhaps there is a pin compatible IC which can be substituted?
The replacement backlight IC arrived the other day. I am holding off on replacing it until I get a few more measurements. Not sure it will even work, might be bad/defective, so it would be ideal if I can remove the IC without cutting the pins. I'll see if I can try the solder bridge/chipquik methods perhaps.