I can answer some of these questions for you out of my own experience. But like @anon19508339 said, the premium plans are not provided by us, so you may want to contact them to be sure.
Yes.
Yes, although that’s also true for free hosting.
API access to external services is quite open on both our hosting and iFastNet’s premium hosting. We only restrict access to some social networking APIs (to prevent spam bots), but no such restrictions are present on iFastNet.
Yes.
Yes. You can create as many custom database users as you want, and choose which permissions they have to which databases.
You can do this on both free and premium hosting. We don’t restrict access to external database hosting services.
The free hosting databases are not accessible to tools running elsewhere, so you cannot connect files on other web hosts to our free databases. But that is possible on premium hosting.
Point the domain to the nameservers of your premium account and add the domain to your account. It’s no different from our hosting.
The only notable difference is that the nameserver validation is not present. So if you would prefer to setup your domain name and website before changing the nameservers, or would prefer to not change your nameservers at all, you can do that on premium hosting.