I just learned that, according to Dolby, if you set speakers to large the LFE is supposed to be mixed in with the L/R: https://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/38_LFE.pdf
I know that's not the case with Plex when you set the output to 2.0, and I guess it could be implemented differently in different receivers, but it's easy enough to test with a laptop and REW. Send a signal on channel 4 (LFE) to the receiver with the speakers set as large. If you get signal out then you know it's retaining the LFE. You should also be able to confirm whether it's keeping it at +10dB or not.
In the case of panning, I would assume that the mono LFE channel would be added to both mains, meaning that it doesn't move? What you're saying about panning would be true for below-crossover frequencies that stay in the L/R channels though.
Also adding to Ultra's comments above, I believe that adding a sub lightens the load on the amplifier. This is because receivers have active crossovers (i.e. the crossover sits before the amplifier stage). This means that, with the speakers set to small and the lower frequencies redirected, you only need to amplify from 80Hz up (assuming an 80Hz crossover). Given that it takes twice the power to amplify 70Hz as 80Hz to the same level*, and so on all way down to 0Hz in 10Hz increments, dropping this range from the amp helps the amp play louder, with less distortion, across the remaining frequency range.
*I think that's right, I'm pretty sure it is.
I'll experiment with REW when I get the chance to solve this! I was mucking about it with on my desktop but I could only see L/R output. Presumably as I had headphones in and so it wouldn't let me output a LFE?
As to the load, yes it's true but given the Neumann have a specific amp for their 8" (and a safety cut off mechanism if you try and overdrive it) at my volume level it may be they are able to cope fine.
It's true though expecting the Neumann to do well at 20hz is asking too much. Unfortunately from memory the lowest cross over the Yamaha does is 40hz. That said the frequency response for them from 40hz up is flat.