Sub Domain Required?

No, a subdomain is not required but is recommended for SEO purposes. Check out our knowledgebase article on the subject: https://support.idxbroker.com/customer/portal/articles/1919242

Yes, but can the results appear on the root domain rather than a subdomain?

No, the closest to this would be by using the IMPress Listings plugin to import the active featured listings from the account. The actual feed is hosted on our servers to provide the fastest search capability possible.