Sometimes people don't get notice about combo but it's conducted on the day. You should prepare to do combo and bring the spouse regardless of whether interview letter mentions I-751 or not. On the day, tell security you're applying based on marriage and have ROC pending.
You should upload I-797 for I-751, not for N-400. No it's not too late to request combo in additional evidence.
These are meaningless estimates. You can roll a dice and have a better prediction. People worry about it all the time, but you won't find anybody saying that estimate was close to when they got decision. You may get naturalized and it will show 6 months to decision.
Being employed is not a prerequisite or requirement for naturalization. If asked, tell the truth on the day of interview. This has 0 effect on your application.