I did my own small test in Windows 10 with a two step script:
Export field contents
show custom dialog
It successfully opened the PDF.-- but this is with a WIndows system not Mac. I'd expect the same results in either system however.
Is there any chance that the file opened in the back ground or would open once you closed the dialog box?
Thank you for trying
I tested your file on a Mac running OSX 10.11.6 and it runs as expected both ways with your fm12 insert. Inserting a different file gave problems. I think it has something to do with the fact you seem to have hardcoded the actual file name in your scripts. I reset your Set Variable step to "Get(DesktopPath) & GetAsText ( Container::container )" and it worked fine.