It's unclear from your post which link key you are using. If it is the standard ZigBeeAlliance09 TC link key referenced in the post, then it will likely not work since Tradfri uses ZigBee Light Link. ZLL uses a different link key to the ZigBee Home Automation / ZigBee 3.0 well known shared key and The ZLL master key is not publicly available (although there ...
I was able to connect to CC2531 via serial by treating it as a CC2530 and ignoring its USB: I flashed CC2530 firmware to it, then connected the serial lines as you would to a CC2530 -- to the P0.2 and P0.3 pins, and powered it via the Vcc/GND pins.
Tested by running Tasmota 8.2.0 against it and pairing one endpoint. Everything worked as expected.
As I'm ...
In general, if it is connected to mains power, it’s a router, whereas if it is battery-powered it’s an end device. Not sure this is 100% true, but probably very close to it.
This page shows it is an FFD (full function device), so it can act as a router.
The pin out for the debug connector (https://lemariva.com/blog/2019/07/zigbee-flashing-cc2531-using-raspberry-pi-without-cc-debugger) looks to be SPI not serial so unless the board exposes the raw Serial pins I doubt this will be possible.
But could you not just use a Raspberry Pi (even a Pi Zero W) and run the code on that?
XBee seems to support mesh networks almost out of the box.
Interfacing with a Raspberry Pi or similar computer system may be ...