Is there any way to link two separate posts, one in WP and one in Discourse, so that the comments from the Discourse post are shown on the WP post?
Also, I use shortcodes for images and stuff on WP, which are transferred as text to Discourse. Any solution for that?
Last question, If I have a WP account, but I have not created a Discourse account for it, and am publishing a WP post with “Post to Discourse”, who is the author on Discourse? What is the best way for my WP authors to publish on Discourse through WP?
Yep, that’s what the wp-discourse plugin does 
See posts on blog.discourse.org for an example.
Best workaround would be to use only summaries in the Discourse topics instead of mirroring the entire post.
I think it’ll then default to @system. Let me know if you put it to the test 