All great questions.

Here's where my hazy memory gets even hazier. ISTR that, with a bum filter and two internal drives, I would occasionally get an error message that seemed to refer to a third floppy drive (which was not hooked up). This seems to suggest that the OS/ROMs have support for a third floppy. But I've never tried this out (maybe others here already know the answer to this; please chime in!). In my case, I just clipped out the filter altogether, because I had all the drives I needed already hooked up to it internally. I never attempted to connect anything to that external floppy connector because I feared it would be a Very Bad Idea. But I don't really remember for sure.

I might have some notes on what I did. I'll go rummaging this weekend (maybe) and see what I find.