Explanation of the scheme (this is the maximum option that I can imagine):
2 or more Discourse multisite installations. Discourse 0 is a common SSO provider for other Discourse (in the same installation or another). It is also a SSO provider for the network “WP Multisite host 2” and individual network sites “WP Multisite host 1”. Some single WP or / and some Ghost websites.
Arrows in the diagram denote SSO
Is it possible with standard plugins? Any problems?