I have a sign permanently on my doorbell saying "Baby Sleeping, please don't ring doorbell". I've had it on there for 5 years (since DS1 was born) LOL! The reason I kept it is because it seems to keep away salespeople and JW's, etc. I think it's fair what you have written for them

As for drop-in visitors, maybe you could say something like "Hi all, I am currently working and am not available to answer the door. Please call me later and we can work out a time to catch up properly". This way it doesn't make someone feel bad for dropping in and doesn't sound rude (I don't think!).

HTH