Now that you have a little taste of panic. You can start preparing for the next disaster.I was charged $17 for everything from UC Davis, and the vet visit was $31 including the blood draw, so my total came to right around $50. Not bad at all.
Seeing as you are short on space I would consider getting some of those cardboard pet carriers as instant hospital beds. Im sure you could rig a water system easily enough.
I would also make up a chicken first aid kit and spend some time preparing it little by little.
I think bio security is something you may be a little over sensitive about at the moment but that can be a good thing. Just set some rules and find a routine that is livable. You will not be able to protect them at all times as Wild birds and the wind can blow just about anything into their path.
There will be lots of things you cannot know or control. Im sure visitors are fine if you take precautions. Dont be afraid to ask if they have had contact with other birds today. You may want to provide some slip on sandals or something and bottle of purel for them to use before and after if they are to actually interact with your birds.
You will know better now some questions to ask when getting new birds as well.
Just like anything you can go overboard and still have things happen. There is a lot you can do but you short of vacuum sealing your coop cannot be 100% safe. There will be people who may visit and not even interact with your chickens that can carry in something. Some things are so prevalent in the environment that you simply cannot stop them and have to trust the birds immune system and be vigilant in separating any who show symptoms.