Q5.0.13: How do I start gnuserv so that each subsequent SXEmacs is a client?

Put the following in your init.el file to start the server:

(gnuserv-start)

Start your first SXEmacs as usual. After that, you can do:

gnuclient randomfilename

from the command line to get your existing SXEmacs process to open a new frame and visit randomfilename in that window. When you’re done editing randomfilename, hit C-x # to kill the buffer and get rid of the frame.

See also man page of gnuclient.