I was pondering this question just last week.
Firstly, I think for a second floppy connector, you need only clone the existing connector but replace the new connector's enable line with the other enable line. You'd have to (carefully) solder a wire directly to the SWIM to get that signal, as it's a no-connect on the board. I was wondering if that's all it takes - if the OS/ROMs will poll that drive, if one is connected.
As far as J-18 goes.. no idea, but I will speculate. I was thinking it might be a diagnostic, development, or debug port.. not intended for product use. Allows attachment of logic that can drive the GLU to dump its state, do some sort of tracing, etc. No idea. I need to read more about what GLU does and what it is.