I only have some minor disagreements with your post, Lancie. Most have to do with the in panel wiring. First of all, it is impossible to completely separate analog wiring in a panel, if for no other reason than you have to connect to I/O module terminals only inches apart. It isn't practical to use shielded cable on internal panel wiring for 4-20 mA loop powered devices, for example, because of the conections to the power supply.
In our panels we use separate wire duct for analog and 120 VAC wiring, but the separation is, reallisticly, minimal. We don't use shielded wire on the internal analog connections from the field terminal strip to the I/O card, or from the power supply to the field terminal strip. We haven't had problems with noise, EMI, or RFI in the panels ever, and that's with nearly 20 years of doing it this way.
I don't have a theoretical basis for this, but intuitively it makes sense that length of exposure to the EMI/RFI source will increase the induced voltage and current. I can't imagine that a parallel run of 1" of non-shielded analog 4-20 mA cable and a 120 VAC 20 Amp power conductor will have the same effect on signal integrity as a 5,000 ft. parallel run of these same two conductors.
Again, I can't cite references, but I don't agree with you about separate conduit not being effective. This is partly based on field experience and anecdotal evidence. Besides, EMR/RFI signal strength follows an inverse square law. Using separate conduit is certainly going to impose some physical separation between conductors.
EMI/RFI is a peculiar thing. I'm reminded of a story a crusty old veteran told me many years ago. He built a HeathKit radio (tube based, of course) and it wouldn't give him decent sound. He verified his assembly and wiring, and finally called their technical support. The technician told him "It sounds to me like the problem is, you are too neat. If all of your wiring is in neat bundles and the wires are all parallel you will pick up a lot of interference. Take your bundle apart, twist things around and have them criss cross, and you will be OK." My friend did this, and got many years of top quality sound out of that radio.