Hi, welcome to Discourse meta.
There is a configurable site setting email domains blacklist where you can add a list of such domains.
I bet there should be an already existing importer. But if there isn’t, You can pay @pfaffman to write one.
EDIT: Here is one Migrate a XenForo forum to Discourse
Yes there is. And it is one of the most well translated in Spanish.
Yes, all those themes are free and available for use. Plus much more.
There is no concept of skins in discourse. It’s just themes.
There is a dedicated howto for changing the domain name. You should be good to go.