Good point, the multi-AZ RDS feature is a nice way to get most of the resilience upsides without any additional app complexity. You do double your database cost, but that might be worth it.
Not necessarily. You could keep the reader a smaller size and scale it up only if needed.
"dual az" is a checkbox that doubles your cost for transparent failover; it's different from the read only replica
That’s assuming that you a) religiously test with the smaller size and b) are comfortable that scaling up will work when lots of other people are shifting workloads, too. I usually work on projects where we haven’t wanted to deal with that but that’s a judgement call.