If they are sneezing that frequently, then it's probably best to treat.
I'm sorry but have not kept up with everything that was said here on your thread
If I remember correctly, you have treated with Corid as well?
I'm just brainstorming here - they seem to be o.k. with the heat you give them correct?
How fresh is your food - I know even if you buy a new bag at the store, sometimes a store can have old feed-does you bag have a date on it anywhere, does the feed smell "fresh"?
Any chance something in their area/environment is acting like an irritant -air freshener, cleaning products, aerosols, etc.?
I hate to hear your just really starting out and are having problems. Sometimes that happens when birds come from different sources. Birds may not show symptoms until they encounter some type of stressor - going to a new home, integration, coming into lay, etc.
Do keep in mind that respiratory illnesses are contagious (you know this), depending on which illness they have, they can be carriers for life even when recovered and can pass it along to other birds.