Hmmm... Lots of ideas, here are my thoughts so far. What kind of machine are you looking to exchange with? That might help narrow down the best solution. My ideas here are more general.
Possibly - the functionality of each LT-to-Eth device can be different (one-to-one bridge, many-to-many bridge, MacIP, zoning, etc), so there is no universal answer as to how well this can work for you. Also, to be practical here it would require that your machine "at the other end" be capable of AppleTalk/AFP.
Supported on Macs with built-in floppy drives not running OS X. I use a beige G3 with the latest revision of Mac OS 9 for this all the time, no problem. 400K disks can be an issue because of the file system (MFS vs. HFS) but if writing images using Disk Copy it's not an issue since the disk is never mounted by the newer OS.
I've had issues with how different MacOS/System versions handle this... Sometimes a connection can be achieved in one direction and not the other. Since I've always been able to work it out somehow, I haven't yet pursued why.
MacDrive should enable this technique from a PC, yes. In my experience SCSI backward compatibility is pretty good.
Possible yes, terminal software at either end, use ZMODEM or similar to transfer... Depending on the modem you may have to supply line voltage similar to a phone line for the modem to detect a connection. However here you're limited to the modem speed.
Plausible, but requires the FDHD ROMs to be present in your SE, in addition to the 1.44M floppy drive. However, once this is done, it is by far the easiest method when coupled with the PC Exchange control panel (req. System 7+).
Yes, they exist. I've never seen one in person, so can't speak to their ease of use...
Lots of custom development would be needed for this one. You'd basically be programming the equivalent of a softmodem.
Your train of thought here could be simplified... A null-modem cable between the SE serial port and another machine's serial port, with speed settings at both ends that the SE is happy with, using terminal software and ZMODEM as in the modem solution, would suffice here.