I've heard that Google deranks content it believes to have been duplicated, and I don't know if that's really true but I've been paranoid about copying my existing Medium content to my new blog; if it's true then Google will certainly give precedence to Medium.
It is true yes, that's why the generated blog have a canonical link to medium so that the SEO isn't impacted. So you won't get any SEO for the articles that have been created on Medium but if you create one on your blog first and then import it on Medium (using the import tool, not copy pasting the content), Medium will be nice and put a canonical link pointing to your blog.
Unfortunately, it's not possible to set a canonical link on Medium a posteriori.
I don't believe your blog would get any improved SEO benefit from having canonical links to Medium - IIRC, Medium would get the juice.
Furthermore, I don't know how Google handles cross-domain canonical reporting, but if I were writing a search engine, I would treat it as highly suspect.
You've got it backwards. There will be a header in the Medium post saying "the original source for this content is on this other blog, I'm just a repost". Google does pay attention to that and ranks the canonical source higher.