In my development environment, this works if I do not urlencode the redirect_to parameter. Whether or not this works may depend on your WordPress site’s server configuration.
It’s theoretically possible, but would require a Discourse plugin. I’m not sure that it would be worth the trouble. Adding a WordPress SSO login link to your forum should take care of most cases.
If you want to automatically create WordPress users when they are created on Discourse, a Discourse plugin could be written that posts to the WordPress REST API users/ route.