Is the trip necessary? Airports is place of elevated risk for being detained if out of status. In may never be an issue, but nobody can give guarantees.
N-600 isn't redundant. Many years later USCIS / DOS / SSA / employer may not be happy with just US passport and demand to see other proof of US citizenship!
See all reasons for N-600 here:
They must update the address as @Crazy Cat outlined above. Better late than never.
Most likely, updating address late won't be an issue. In theory, failing to file AR-11 is a deportable offence.
I'm very sorry about your mom's situation...
What country is your brother citizen of? Can he come on ESTA or visitor's visa? When was I-130 filed? Is mom a GC holder or US citizen?
You cannot get approved for citizenship within 90 days of early filing window.
So USCIS typically would schedule for interview past early filing window.
3 year rule is a more complex process, so may take a little longer too.
1) There's no form 1-29F. I think you mean I-129F
2) It just asks whether you met in the last 2 years in person. It could be yesterday. No problem, you don't need to wait 2 years before filing.
It's just the way it is with I-90. Some get processed super fast, others drag.
Can you make an inquiry? Are you within normal processing times?
https://egov.uscis.gov/processing-times/
It looks like 26 months it is currently...