New Post: merge PDF pages into a single PDF

I have a PDF created by Doc1, so it's really several separate documents. I can split it apart just fine into separate pages (e.g., -sOutputPath=\[path]\P-%06d.PDF) but trying to combining subsets of pages is giving me the famous -100 error:
Ghostscript.NET.GhostscriptAPICallException: An error occured when call to 'gsapi_init_with_args' is made: -100
at Ghostscript.NET.Processor.GhostscriptProcessor.StartProcessing(String[] args, GhostscriptStdIO stdIO_callback)
at Ghostscript.NET.Processor.GhostscriptProcessor.Process(String[] args, GhostscriptStdIO stdIO_callback)


The command args are:

• -sDEVICE=pdfwrite -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -sOutputFile=\\server\path\nnnnnnnnnnnnn.PDF
• -f\\server\path\P-000001.PDF
• -f\\server\path\P-000002.PDF
• -f\\server\path\P-000003.PDF
• -f\\server\path\P-000004.PDF
  

I can build this command string and run it fine through GhostScript in a command prompt (gs v.9.16)

Any hints? My GhostscriptVersionInfo object is reporting gs 9.16 just fine