That is strange, never seen that one before. It almost seems like something is holding the entire computer in a state and not allowing the clock to to roll forward and the processor to proceed to the next instruction. Just a very uneducated guess on my part.
Your voltages look decent. Those are constant, correct? Since the video looks stable and sound is loud I'm going to guess that your analog board is okay.
I'd look back to the logic board and floppy drive. Try unplugging the floppy drive and give it another go. Still same symptoms with the floppy drive unplugged?
PRAM battery? Completely dead and gone is okay. Healthy is obviously good. Some people (myself included) can get away with a AA battery @ 1.5 volts. HOWEVER, a battery much below 1.5 volts, but above 0.0 volts may be leading to corrupted PRAM. Remove the battery all together and try starting up again. Still the same symptoms with no battery present?
You mention the mouse is not responsive. Unplug it, and use a can of compressed air to clear all the ports on the back of any possible debris, along with the keyboard connector up front. Power up without the mouse or keyboard plugged in. Still the same symptoms?
How does the logic board look? Use the compressed air to clean it of any dust. In rare scenarios, the dust can be causing small shorts, leading to strange anomalies. While it is out, go ahead and reseat the two ROM chips, as well as any other removable chips. Sometimes corrosion forms in the sockets causing a bad connection. Assuming the board isn't rusted out, a simple reseating of these chips can usually clear enough corrosion for a decent connection. After reseating the ROMs and any/all other components on the logic board, go ahead and try firing it up again, still with no mouse/keyboard/floppy drive.
If it finally does work, slowly add things back one at a time until you either have a working system, or found the component that caused the failure you are seeing.
If none of this advice helps, then you may need some more advanced troubleshooting techniques. Good luck