Full-duplex should work. You already mentioned the MOD Duo which runs Jack full-duplex on a board with an A20 SoC. It also works on the RPi and I've used Jack in full-duplex on a Cubieboard (A20 SoC) successfully. In order to make this work I can recommend doing the following:
Don't use a graphical environment, try to get acquainted with working on the command line.
You will probably need at least a preemptive kernel or even a real-time kernel to make full-duplex work.
Also the 1818VSL used to have issues with Linux unrelated to armhf, not sure what the status is now but make sure you use a recent kernel. And bear in mind that KXStudio has no armhf packages.