I too am trying to do roughly the same thing as OP, except that I am using the
debian-502-amd64-netinst.iso and am installing it off of a USB stick.
My issue is that when installing VIA the graphical methods, AFTER it has asked me about my language and location, it always tries to mount the CD drive and tells me to insert the CD when it does not find it.
Is there a way to force it to check the USB stick instead of the CD drive for the files it's looking for.