You cannot program the mesh devices with the old debugger shield because they don’t fit, and also because the mesh devices have a different processor, not a STM32. You also cannot use a ST-LINK/V2.
The Particle Debugger (CMSIS-DAP) is what you need to program or debug mesh devices. You can probably use something like a Segger J/Link as well, but the Particle Debugger is inexpensive and will be the supported solution. Right now, the documentation is not complete because it’s awaiting debugging support in Particle Workbench.
I’m not actually sure you can write the external flash from SWD. I’ve only done it from DFU mode.