This is super optimistic. The portal's estimated time to approval is largely incorrect. I can have a better prediction by rolling a dice. With I-751 pending I'd say more realistic estimate for approval is 4-6 months if your N-400 case goes fast.
Each case is unique. There's many variables affecting approval timelines.
Building a strong case cannot hurt. Building weak case can add time due to RFEs. Most of the time the case sits in the queue. 1.5 years of wait could have been a result of external factors you cannot control: backlogs, inexperienced IO etc.
When the child is born, upload the birth certificate. If anything it can speed up the application. You must disclose all kids on N-400. Disclosing kids at the interview or any new information probably increases the changes of "decision cannot be made". USCIS needs time to understand your case. They can either spend time before the interview or after. This kind of relates to "heavy" vs "light" application. In my opinion, USCIS will need the infromaion regardless. And either they call you in for interview sooner to issue RFE / gather more info, or they examine everything beforehand.
It's impossible to split statistics for 3 year vs 5 year eligibility cases as USCIS doesn't make a distinction in their reports.
I wouldn't think so.