The nostril is looking much better with the flushing. Infectious bronchitis is common, the most common respiratory disease in chickens. Chickens will remain carriers for up to a year after recovery, so good not to add any new birds until then. There are 3 types of Newcastles, and the exotic NCD is not found in the US. That normally will wipe out a flock, and is only a problem in a few countries.
IB is indeed responsible for many egg shell problems. Have you ever seen a wrinkled egg shell? Thin shell or odd shells can be due to IB. Are your chickens getting layer feed plus some oyster shell on the side for extra calcium?
As my hens got older, I would see more odd shells, soft eggs, calcium deposits, soft ends with a hard shell, malformed shells, etc. Here is a good article about egg shells:
https://www.backyardchickens.com/articles/common-egg-quality-problems.65923/