1) Wherever possible, get the mono mixes of the music. They're frequently quite different, and usually better sounding. The monomix of Piper, for instance, is worlds better. If you're listening to 60s music, this is the best option by far, even when listening to "both" channels.
2) iOS has the option to force mono. Rather than put it in the Sound menu, it's in the Accessibility menu. Dunno if they ever put it into the iPod software or not. It is definitely an option in the
Rockbox software. Personally, I have not had an iPod not running Rockbox in years; I have trouble understanding why anyone would even
want a player that can't handle .ogg, or especially .flac files.
3) You can get an 1/8" stereo to 1/8" mono converter at Radio Hut or any similar supplier of audio dongles.