Fine to Add Canonical From Secondary Domains to Preferred Domain
John says it’s fine to use a different domain or set of URLs for offline marketing and to implement a canonical pointing to your preferred domain.
Content in Iframes May be Indexed on the Embedding Page
Pages embedded within an iframe on another page may be indexed as content on the embedding page as it will be seen when the page is rendered. You can use X-Frame-Options to prevent browsers from embedding a page which Google will respect.
Mobile URLs Won’t Need to be Included in Sitemaps for Mobile-first Indexing
It won’t be necessary to include mobile pages in sitemaps for Mobile-first indexing. You should continue to include on your canonical desktop pages. Google’s goal is to keep everything as similar as possible while indexing content on mobile pages.
Google Automatically Chooses a Preferred Domain Based on Multiple Signals
Google uses many factors to determine the canonical domain including redirects, canonicals, internal links, sitemaps and backlinks, so choosing a preferred domain in Search Console is not critical.
Backlinks are reported in Search Console on the canonical domains
If you canonicalise domain variants (www/non-www/http/https), all of the backlinks will be reported in Search Console under the canonical domain.
Google Chooses HTTP if the HTTPS contains Insecure Content
If Google finds insecure content on the HTTPS version of a URL, it is more likely to choose HTTP as the canonical version.
Separate Non-WWW/WWW and HTTP>HTTPS Redirects are OK
It’s OK to use separate redirects for Non-WWW to WWW and HTTP to HTTPS which can result in 2 steps. Ideally you would redirect in a single step where possible.
Prevent Test Site Being Indexed with Canonical
John recommends using a canonical to the main site, although he says that it’s possible for both to be indexed.
Mobile Sites Don’t Need Sitemaps
Separate mobile sites should be canonicalising to the desktop page, so you don’t need to submit them to Google via a Sitemap, but it’s still worth adding to Search Console.