Is the failure to report observations about a building under construction—which may indicate a potential danger, such as rusted or bent rebar—considered a religious responsibility, especially given a lack of engineering specialization, and does the failure to report lead to accountability before God if harm occurs?

You are not required to do anything because of doubts concerning buildings. This question falls under the category of waswasah (whispers/obsessive thoughts). You must completely disregard these waswasah, stop preoccupying yourself with them, break off from continuing with them, and refrain from asking about them. Supplicate to Allah to grant you well-being from them.