Links so far in my research worth keeping (no solution yet)
a) recovering a runtime solution – hold down Ctrl-Shift while double clicking application icon and hold them down til you see the dialog
http://www.filemaker.com/help/html/fmpa_tools.24.19.html
b) importing data into upgraded runtime
http://www.filemaker.com/help/html/fmpa_tools.24.17.html#1028635
c) starting runtime solutions
http://www.filemaker.com/help/html/fmpa_tools.24.18.html#1028687
d) naming a filemaker solution and this mysterious file extension
http://www.filemaker.com/help/html/fmpa_tools.24.15.html

Q’s for self: in the docs above, there is

• a runtime app file and
• a solution file – so what is this “solution file extension” and where is this file with this extension?

Sure enough – look for this renamed .fmp12 file or solution file – this is the database file. In fact – the person who “packages it up” using the Filemaker advanced designer tool was prompted to make this extension change so write it down along with the special password or packaging code.