I recently built a full duplex node using an Allscan ANF101-M with two RT85's attached. I don't like the long squelch burst of the RT85 receiver, and simpleusb has no delay line to compensate for that, so I used usbradio with rxsquelchdelay = 250 and rxondelay = 12, because otherwise, there is a slight pop at the start of received audio, as well as a squelch tail at the end. This means that responsiveness is a little more sluggish than using simpleusb, but it's worth it to me to not emit a squelch tail when not using a radio that puts out a suitable 55 Hz STE, or using DCS. My current favorite non-duplex HT (TYT MD-UV380 running OpenGD77) doesn't implement DCS properly, so that wouldn't fix the problem with that radio anyway.
While testing simpleusb, I set the directive duplex3 = 28, which gave about the same level directly through the sound fob's microphone playback as through ASL3 when duplex is set to 2. Then, just for fun, I tried the same thing in usbradio.conf, as it is referenced in the template, and it did nothing.
Granted, you would almost never want to use this setting with usbradio, especially if you have a flat radio, because the PL would come straight through the other side, and the resulting audio would sound sharp and tinny.
So, if it's supposed to work, it doesn't, and if it's not supposed to, then it probably shouldn't be in the template.
I read this a couple times and couldn't quite get to a specific question so sorry if I'm not answering your issue directly.
I have a number of RT-85's and use DCS in them for allstar nodes of various types. I never get squelch bursts or tails or whatever we are calling them today. I also use DCS on the local HT's accessing the nodes which eliminates me hearing anything when my node is done transmitting. Since all the CTCSS/DCS "work" is being done in the RT-85's, I use simpleusb. Easy peasy. And DCS even works right with Yaesu believe it or not. Hope that helps.
The specific reason I am using usbradio instead of simpleusb is because my preferred HT doesn't send the required 134.4 Hz tone burst at the end of transmissions, the way any DCS radio should. Thus, it's out-of-spec. Yes, I could use another radio. No, I don't want to.
It also doesn't respond to those properly. Plus, I don't like the aliasing that simpleusb has on transmitted audio, which usbradio does not. Using usbradio with rxsquelchdelay fixes all those problems, no matter what radio I am using to talk to my node.
But this isn't really the point of this topic. Point is, a probably very obscure to most feature, duplex3 = x, is not working at all with usbradio. I don't even need it, and what I have works fine, but I'm just pointing out that it's referenced in the config file, and doesn't work in usbradio, but does in simpleusb.
Looking at the code, duplex3= in both simpleusb and usbradio is just a boolean value of 0 (off) or 1 (on). Making the value higher does nothing. All that does is, if true, hook the audio into an alsamixer interface. Other parameters may influence what happens after that.