[Hummingbird] HB and Smush problems

Dear support team,

I've enabled HB on my multisite network curly-de-provence.fr and I have a few issues.
I activated support access.

1) I haven't been able to find the "Critical CSS path" option in WP admin, could you please point it out to me ?

2) I'd like to have more information on asset optimisation in multisite. Here, it seems that nothing has been triggered (grey spining icon) and that sub-sites recommand not to change anything (orange icon)

3) When I activate the Smush CDN, images are unavailable on the website. There are no errors in the console.

I granted support access, so feel free to have a look around and to make any suggestions towards improving site speed.