I'm pretty sure the Plus came with an 800k floppy, as its identical to the 800k internal floppy drive in my SE.

Here's an update:

My Mac Plus boots fine with the floppy emu (no surprise here)

I found an old Symantec emergency disk, and the Mac Plus boots fine with this disk when attached  using an external 800k floppy drive.

The Mac Plus behaves as previously described when using the Symantec disk in its internal drive (whirring sounds, non ejecting).

Holding the mouse button down while booting has no effect.

I also attached the troublesome internal 800k drive from my SE, and it just hums (makes no other movement or sounds).

Does this point to a bourns filter problem on both internal drives? Any way to confirm with a DMM?

THANKS!