I have a silk liner. It's like the one you describe - a ''sock''. It's light, though, so I sleep in it when camping. It's not too warm even during the hottest night.
I'm walking in March, so I'm hoping that I won't have to worry about bed bugs. I don't plan on treating my gear with anything. I hope that's the right decision! I guess I'll find out...
In addition to the liner, I'm bringing a down throw (smaller than a twin blanket, but big enough to cover me completely). At 715 grams, it's lighter than my sleeping bag and I think that it will be pretty versatile. If it's extremely cold, I plan on putting the blanket inside my liner to make a sort of sleeping bag. Its main selling point: I already own it. I wish I could buy a fancy high-tech down sleeping bag, but the ultralight ones are well out of my price range!