Status updated like many here to "case is taking longer than expected to process".
I believe when it gets to NVC it will move a lot faster until then it is going to be waiting game.
Of course it may vary case by case or field office. the only supporting document they asked to provide was birth certificate of kids. scanned them and approved I751 then moved on to N400.
Hope it helps.
IRS Transcript should be fine. In the past I had submitted and had no issues. I believe notarization is for the foreign documents, such as birth certificate and etc...You will need to bring original copies from what i remember. Are you applying online or via mail?
Yes, we submitted N400. Current timeline shows 7 months so it could be sooner or later. I believe it is a good sign, perhaps they will update I-751 and N400 concurrently.