Can I use MainStage without owning Logic?
If you buy it from the AppStore, YES
For a $29 download, does MainStage also come with all the content (sounds, etc.) for its built-in instruments, or is it assuming you already have those b/c you presumably own Logic?
All the instrument and FX plugs are included, and a (massive) additional free download within the Mainstage application will get you all the extra content.
This question may need its own separate thread, but: Ideally, my keyboard setups for a "concert" may need as many as five VSTs per song
Caution: Logic/Mainstage will only work with Audio Unit (AU) plugins, NOT VST. Most 3rd party plugs are available as AU plugs, but you need to make sure you have installed them.
Has anyone tested this more-complex and memory-taxing sort of a setup out in a live situation?
Yes, and memory/CPU issues not withstanding, Mainstage generally works extremely well for this.