I just picked up a Macintosh SE with a 800k floppy drive. As I don't have 800k floppies laying around nor a way to nor a way to read them on a modern(er) computer, could it be possible to use a 1.4M capable manual-inject drive (like the Mitsubishi MF355F-3592MA) instead of the usual Sony MP-F75W ?
(other question, where could I find a Sony MP-F75W for a reasonable price instead of the price of a whole Macintosh ?)
The original SE (with 800kb drives) lacks the hardware or firmware to support high density disks or 720K MFM disks. The upgrade kit M0242 includes new firmware ROMs and a new floppy controller called 'SWIM' which adds MFM encoding capabilities. The original IWM chip can only handle GCR encoding on 400k/800k disks.
You also asked about manual-inject drives. The older Mac cases were designed for auto-inject drives and actually do not provide a wide enough opening to push the disks in manually. On top of that, the signals on the floppy drive cable changed from the auto-inject to the manual-inject models, causing compatibility issues. Different mounting methods were also used, which makes it unlikely that a manual-inject drive could be satisfactorily mounted in a SE.
The solution most often adopted for getting software onto 800K disks for early Macs is to use a "tweener". Any Mac with a built-in floppy drive (so up until the beige PowerMac G3, or the PowerBook G3 Wall Street with internal 3.5" floppy disk drive) can write 800K HFS disks. You could also use network file sharing over LocalTalk if your SE already has system software that includes AppleShare (this option requires a file server that can speak LocalTalk—a "tweener" older Mac again—or a LocalTalk bridge to an Ethernet network).