Can I set up an AppleTV with: - one Apple ID for home sharing - a different Apple ID for iTunes Store purchases?
I mean: setting the Apple ID for the iTunes Store will change my Home Sharing config on AppleTV?
Yes, you can set up iTunes to have an 1 Apple ID signed into the store and a second Apple ID used for Home sharing.
You can also do this on an Apple TV. You are able to have multiple Apple ID's saved on an Apple TV and switch between them to login to the store.