I think, Phil, you hit it pretty square.
Nonetheless, I still think that on the channel strip, all of this volume and EQ and, perhaps, limiting or compression or added internal effects (if it’s effects send, you have to boil it down to a stream and send it out and that requires quantization) can be done with a very wide word for each sample. (I forgot, you guys call it “bit depth”, so when an EE says “word width”, the audio guys say “bit depth”.) In the olden daze I would say 32-bit float, but now I see no reason a digital board or a DAW or some other device manufactured in the 21st century can’t be doing all of the internal processing with really wide words, like 64-bit doubles. Do your gain control, your EQ, your compression/limiting/gate, your added reverb, pitch correction, whatever, even the buss, all of that should be done with 64-bit doubles until you have to export it out to AES/EBU or S/PDIF or a D/A or write it to a sound file that’s fixed-point format. Or a codec like MP3 or AAC or FLAC or whatever.
Then and only then will quantization be necessary. Then, at the quantization point, is the correct place to add TPDF dither and to employ noise shaping around the quantization operation.
Even if it’s just a volume control, it’s sorta kinda inexcusable to not dither it after the gain scaling, and if everyone in the signal chain were keeping their noses clean, then we shouldn’t have to predither the input for any reason.
But some gear is old. And not everybody keeps their nose clean. Some gear, some DAWs, some plugins fall short of appropriately dithering their final quantization operation. Then, if the audio it’s working on already has a little dither to make it dance around the quantization level a little, fine. Let’s do it. But it’s not really an appropriate substitution to proper quantization in the first place.