can confirm, the flash when riding that spinning disc thing is normal

(nevermind you figured it out, the following paragraph would have probably led you down the wrong path) Color signals look like they're shorted to CSYNC with the latter pretty much drowning out the picture information. The CSYNC has no business to be on the color outputs on 12:10 to 12:40 (unless it's a game with a black background and negative video like old Donkey Kongs). (this big 5V TTL signal shouldn't be on the color outputs at all, just the video information in between the pulses, and that should have its black level close to 0V) These signals in the beginning have a lot of overshoot or ringing. Half a volt (even negative) can be considered normal and can be ignored. At least that's what I think, because I see that all the time on working boards too. Can happen with long leads and/or impedance mismatch. If you zoom in with the oscilloscope you can distinguish between noise and ringing - ringing will look the same on every transition like a high frequency sine wave. If it's more than half a volt maybe try a termination resistor in the other end (try with 200...50 Ohms). Just guessing though. Your front porch back porch (I keep mixing them up too) theory is pretty much correct. The monitor sees the low portion of the signal (in the blanking period) as the black reference level. With the video signal having the true black start at around 3V that's pretty much higher than 0V so the monitor gives a white picture with a faded inverted picture on top.

What is the book you are referencing at 23:30?

Your TPG H&V sync frequencies is the same frequencies as the arcade motherboards is H&V sync output signals frequencies, but the only differences are the V&H pulse widths are different? That is usual that the frequencies are the same but the pulse widths are different