A couple of things:

https://fusionauth.io/docs/v1/tech/troubleshooting/#troubleshooting-email has troubleshooting tips worth checking out.

The UI copy doesn't copy the SMTP password, so double check that as well.

Yes.

The only thing you can't create via the API is another API key. You can track that functionality on this issue.

See the tenant documentation for more.

Tenants are separated logically, with users, identity providers, and applications all separate.

If you need physical or geographic separation (for instance, to host users in tenant A in the USA and users in tenant B in Germany, you can do this by running two instances of FusionAuth. You can keep system wide configuration in sync using scripts with the client library.

Yes. You can use the tenant API or any of the client libraries to created, read, update and delete tenants and all their configuration.

Sorry, that functionality isn't available in the UI.

No hard limits, we have some clients running somewhere between 5-10k.

If you encounter any performance degradation, you can open a GitHub issue and we will take a look. We do have some work in plan to improve the UI for this type of scale.

User ids need to be globally unique. You can either provide your own unique uuid or you can let FusionAuth provide them for you. But these values are not tenant scoped.