I'm all for the discovery of the truth, so lets drop the intensity of light debate (even though, all things being equal, the bigger mirror, the more light, even if the center portion that would be collected by a crop sensor may or may not be equal intensities, the extra surrounding information may or may not be enough to affect exposure and light), but lets assume that's all equal... Let's take a 40D and a 5D mark 2 (same AF system)... Assuming the AF points are pretty much in equal or equal like proportion to the frame on the 40D and 5d mark II or at least the individual AF sensor size within the frame, and since the 5D is 1.6x bigger, then that would possibly mean, at least to the layman, that assuming the AF sensor isn't necessarily bigger, but if there were lets say 20 pixels of information per each sensor size on a crop sensor, on a full frame, there would be 36 pixels, hence more information going to the sensor, which allows it to be better in low light, which is the original problem that was in question. Any debates about this thinking?
Yes. First, let's just clarify the difference in sensor size so there's no confusion for others. 1.6x refers to the diagonal measurement. The FF sensor is actually 2.5 times larger in area than the APS-C sensor.
You may realize this but it's not clear from your statements and others may not know. The AF sensor is completely separate from the image sensor (it's usually at the bottom of DSLR camera bodies) so therefore the relative size, resolution, pixel pitch, etc. between image sensors in various cameras has nothing to do with the AF sensors in all those cameras. The AF sensors do not need to scale (in terms of size or pixel size) along with the image sensors.
I'm not really sure exactly what you mean by "more information going to the sensor" but that's not really how phase detect AF works.