I respect you for trying to learn more and understand why things work. I understand your argument but as Brent mentioned you are thinking digital but everything boils down to just basic electronics. Forget about the 1s and 0s for a minute that is the data.
A '1' is nothing more than positive voltage and a '0' is no voltage. They are timed so the data is recognized. Whenever you are dealing with voltage there will be amperage, impedance, capacitance, etc.
Because data can be altered and retransmitted does not mean there is a separate circuit between these. All because they go from one chip to the next doesn't mean they are not effected by basic electronic principles. They are still part of the same circuit.
The only way to physically break that circuit is to send something by a medium without conductivity such as wirelessly or fibre. Just like a ground loop could affect every device in your house it is about how well it is isolated. It is all connected somehow!!!
In any system these properites can be measured. In audio you will often see distortion measurements, crosstalk, etc. Same thing with integrated circuits just different measurements. In fact when they build a chip, the number of layers and size of the die will be a factor determining the capacitance of the chip. Between the layers sometimes they use an insulating material to reduce the capacitance, of course this costs more. The more vertical layers helps with capacitance but is more costly to build. The fewer layers the cheaper it is to make.
Basic electronics didn't disappear because of a whole bunch of parts compiled into a chip. Chips have electrical properties as well and is part of their design.
Since you are interested in how things work I think you need to bring it to a more basic level and stop thinking about the data. Hold off on trying to read about HDMI and HDCP that is the data. Do some searches for impedance and capacitance in integrated circuit design and high speed signaling.
This will give you a better understanding of why the problems happen and everything else is just the data.
Hope this helps.
Bob
Ps. TMDS is encrypted and a one way stream. There is no feedback on the TMDS lines that would come from the DDC channel. I think you know that just making sure. It doesn't matter anyway that is just the data:-)
