I have an awesome demo I give running a complex stateful workload across cloud providers to show off the system that I work on. What I have learned from giving that presentation many times is that while it is nice to say you can run cross cloud, for most workloads you should just pick one cloud, and be able to move to another provider if you ever need to.
No, not unless you are someone like Netflix. Usually you can configure multi-region failover and such and that will keep your things running. It is more expensive but for most use cases I think the cost is still less than the dev time/complexity of setting up multi-provider workflows and the inevitable duplication of resources (which is part of the cost of multi-region anyway)
No. And there's been a lot of talk recently about multi-provider being the right strategy to mitigate downtime, which IMHO is a farce peddled by expensive consultants. The parent comment is correct - this is why availability zones and regions have been established by each provider.
For the large majority of businesses investing in infrastructure-as-code far outweighs any crazy HA, redundant, multi-provider, whizzbang whatever setup you may have.
You can move 1.6TB between providers in a month for the same price as a single beefy DB server (m4.16xlarge here). That's a whole lot of logical replication..