It wouldn't surprise me if the iTunes library just wasn't designed to be accessed by multiple instances of iTunes at once. It would also actually makes sense..
The read-online option would be simpy sharing the library from the 'main' machine.
You can prevent the harddisk from spinning down in the System Prefs. It's under Energy Settings.
And in case you didn't know it yet, for questions like "Where can i configure option X", System Prefs has a super usefull spotlight-based(?) search feature.
it actually looks like a spotlight..
btw, you would expect the Energy Settings - prefpane to be in the search-results when performing the same search in spotlight from the menubar.. but strangly enough it isn't. (at least on my machine - 10.4.11)