KCD,
If you check on your pool every few days, unless there is a sudden major leak, the likelihood of the water dropping too low is pretty remote.
In theory, skimmers with only one hole should also be connected to the main drain by some kind of built-in valve.. A lot of them had a plastic piece that you could slide over hole so that it would also suck some water from the main drain. I don't think any of them had the ability to switch between only the main drain, or only the skimmer.. Unless you can shut off the skimmer completely, it will just suck air and once that happens the pump will not be able to suck any water from the main drain.
I have a couple of rent house that have 2-hole skimmers that normally have a floating diverter valve. I had no luck at all getting the floating diverters to work even 20% of the time, so I just gave up.. Both of those pools no longer have an operating main drain, and they both work just fine without a main drain.
Thanks,
Jim R.