Hello Penny, I'm Dr. Deb.
I recently came online and see that your question hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
In my experience, the largest percentages of dogs who've been diagnosed with conjunctivitis are not contagious to other dogs; it's considered fairly rare. However, having said that, I have seen a few cases in my career where that would appear to be the case: Multiple dogs in one household, for example, who both developed similar ocular symptoms.
The underlying cause for what might appear to be a contagious ocular condition could be secondary to a bacterial or viral pathogen (such as herpes) but a more common possibility is that a simple follicular conjunctivitis has developed secondary to environmental irritants (or allergy) in a group of dogs that share the same environment.
The dog who experienced a corneal scratch or injury is totally unrelated.
Reverse sneezing is secondary to a long list of possible causes with allergies on that list but wouldn't directly be related to the development of conjunctivitis either.
I can understand your concern that you may be responsible for an outbreak of conjunctivitis from dogs in your care but in those rare instances where it seems to occur, it would appear to be self-limiting. The dogs who have been treated should respond, the symptoms should resolve and that should be the end of it.
I don't believe you need to take any special measures since you're already doing all that would be recommended to prevent the spread of any contagious conditions.
There may be an element of coincidence or bad luck involved here but it's possible that something else is going on. If you're transporting the dogs in your van and you've taken them to a new place to walk or play, then perhaps this new location is actually the culprit. There may be some plant or tree or grasses which are triggering a response for some of the dogs....just a thought.
I hope this helps to reassure you although, again, my apologies for the delayed reply. Deb