1. No, it’s not mandatory, I didn’t even know it was a thing. We just filed our N-400 and that’s it.
2. Our I-751 status changed right away when we submitted the online N-400.
3. I’m assuming the USCIS 100% knows what cases we have pending, it would be pretty difficult to miss that a person has another case pending. When we submitted the N-400 the docs section asked for my husbands green card. We uploaded the extension letter, which in itself proves that we have another case, I-751 pending.
good luck !