They support localized branding and languages.

It enables internationalized domain names with special characters.

Yes, Shopify Partners can assist merchants.

Visitors may see errors or inaccessible pages.

Yes.

Yes, stores work with the default myshopify.com URL.

From Settings > Domains in Shopify admin.

The custom domain may not be set as primary yet.

Yes, domains can redirect to external websites.

Yes.