Havent tried this but my friend said only way to share music from the computer and on itunes is to create another log in and have access to itunes. plug phone to new created account.
If you keep all your music/media on an NAS you can share it amongst all your computers. Add whatever files you want to the iTunes library on each machine and sync to your phone the way you usually do.
I'm not a copyright lawyer. The purpose behind Home Sharing is to share your library out to up to five authorized computers. It's not really copying from one ID to another since you are authorizing that other computer to import / play your music exactly how Apple designed it, so I doubt there are any copyright concerns.
My PC is set up to automatically import anything from my wife's Mac because my PC as an offsite backup. Her Mac backs up to a Time Capsule but a fire/flood/theft would probably take out all three.