Just my $0.02 on the upper respiratory symptoms:
Had this happen suddenly to my old flock, birds all less than 1 year old. Brought in new birds carrying URI.
Over night symptoms began about a week after bringing in new birds. Came out in am one pullet with eye swollen shut and multiple others with bubbles in eyes, wheezing, sneezing. Researched and treated with Tylan. Only a couple of my birds made it through the epidemic unsymptomatic. Treated them anyway. Gave Tylan orally to each individual bird, dosage based on weight, twice a day. Got full improvement from all birds except one poorly cockerel who was extremely late to mature and generally a poor bird, he didn't make it. None of my laying pullets who showed symptoms layer while showing symptoms! They all began laying again about a week after getting better. So, IF your girls are of an age that they'd still lay during winter then they're probably not because of the URI. Most of my birds lay completely through their first winter and then molt and take the winter off each following winter. If your birds are on their second winter I wouldn't expect eggs until days begin getting longer again. For your young pullets that should be beginning laying, I doubt they will while ill. I don't personally know what the outcome will be if you never treat this URI, but I doubt it would be good.
In regards to becoming carriers, I believe it depends on the type of uri they've got, there seem to be a few different kinds, some being more serious than others. After my flock's URI I bought "test birds" I knew they were healthy, and they were a pair of a breed I wanted to add to my flock. I integrated them directly in 3 weeks after ALL birds were healthy and showing NO symptoms. Test birds didn't get sick and neither did any birds I added after them. So my birds weren't carrying any contagious uri permanently.
Hope this rambling helps.