The reason you might want to use scheduler would be if you had a variety of different programming needs during a single hour. An example might be, auto scheduling news/weather, then some easy listening music to the bottom of the hour, then a change in music format at the bottom of the hour. It could also add in station breaks/promos etc. although you intend to do that manually. I think in your case, it might be easier to just load an itunes playlist into DJ. It could be a smart playlist that is limited to an hour of music from whatever source, be it shuffle or something else. While it is technically possible to manually build an hour with the scheduler, it's quite a few steps to navigate to that and do it. It's really meant to just run in automatic mode. If you're manually doing a show, I think it's more awkward using scheduler than just loading an itunes playlist in DJ.
Doing the mic work is best in the hardware mixer. If you don't have one, all I can say is I highly recommend getting one. Even an inexpensive one. What I typically do is turn Auto off and let the song finish, and then I start talking. When I'm done, I put Auto back on, and it starts playing the rest of the music program again. With a mixer with faders, you could also fade the music to a level you want and not rely on the ducking function. Although, I usually do it the duck key when I do a voice over. Nice to get a consistent level that way.
DJ has a Line In input you could try and use, but it's painful. There will be a latency in the audio that will make it difficult to monitor, as the slight delay causes the brain to get confused, and it will make you talk slower. Quite funny really, but not useful.
The button for that is in the top right of the DJ screen, second from the right.