Mono is fine for most purposes. If you are only recording you in the lecture, that should be fine.
The stereo mic (like mine) would allow you to clip one mic to yourself and the other to an interview subject if you plan on doing interviews or recording live music. I sometimes just clip my lavs to a music stand or something to capture the house sound in stereo.
I do not have one in front of me, but it has on-screen meters and I would be very surprised if it didn't have manual gain controls as well as automatic.
So long as the giant squid pay attention, it doesn't matter. All seriousness aside, mono for voice is all you need. You can later mix the single track with music et al. Mono takes less space, too. Why do you need two mics to record a lecture anyway . You need two mics to record a dialog.
Mono is just fine. While we have two ears, we only have one mouth, after all. (And generally, with a few exceptions, all microphones are mono for this reason.)