I'm wondering in what folder you are when you specify PWL file name for a ipwlf symbol? Cannot find any info on this in documentation.
If you specify a "relative" filename (i.e. one not beginning with /) then it will be resolved using spectre's include path. The include path can be setup via Setup->Simulation Files, and if not set, it will look in the netlist directory (which is the working directory for the simulator) - so probably not what you want.
That should work. You can't use UNIX env vars in the Include Path in ADE though (I've not re-tested this, but there's a CCR on that).