Is this what I’m looking for?

The cheapest way to do all but one thing you wanted to do would be this:

The solution above is way more elegant, by taking away the ability to "mix B with the D so it can be heard [..] on [..] a new pipeline altogether".  To do that, you would also need two Passthrough cores, and the switching on/off would be a slight hassle mainly because you would be able to hear both the Core 1 output through Core 2, and also directly, at the same time, if you have Core 1 also going directly into the headphones. It would also NOT be processing Core 1 through Core 2, which is what you described, which is why it would take a second Passthrough to handle it all correctly. This is true for ANY mixer/router that has the audio path you described.

I'm not sure why you want to process the output of Core 1 through Core 2 (double processing), but I figure you have a good reason.

I have an even nicer solution that'll allow you to have 3 volume knobs to easily control it as well…

This will also give you a fader you can select (in the top of BreakawayOne) for the amount of Core 1 output that you want mixed into the input of Core 2. Everything is the same as the flowchart above, except instead of having Core 1 going into Core 2, via Core 2 Input 2, you would have a "Common Monitor" (which has a speaker controller) with Core 1 HD Output patch point selected, and have the Common Monitor output to Pipeline C.

The only down side is that you'll have a little bit more delay since Common Monitor has to output to a soundcard, and come back in on a soundcard.