Click the link icon to the left of the photo icon and add it as an attachment. You can't do CB Gallery links from CB Activity at this time as it only allows uploads there since CB Activity already has a link feature. You should also be able to just add the youtube link directly in the post and it should automatically attach it unless "Embedded Links" is disabled in Activity > Links > Create for that stream.

When using the attachment feature (the link icon) and supplying a YouTube video URL it will grab that videos title and description same as it does with page URLs and their metadata. It should already link to the video directly. The below is what it should look like. Beyond that I don't understand what you're asking, sorry.

I've no idea. This works fine in all my tests on 2 local installs and our demo site. Ensure CB Activity is up to date. If you're using a custom CB Activity template then change it back to default to see if that resolves it. You can try clearing the URL cache and making a new post with the URL again by deleting the below folder.

/cache/activity_links

Without a means for me to reliably reproduce the issue and no errors on your end there isn't really a way for me to track down why this fails for you. At the very least it should still detect that the youtube URL is a video and output the video embed. It should parse fine as long as the cURL request to the supplied URL replies with a 200 response code.

Will review improving the error output for attachment parsing so that maybe it can be more clear why a URL fails to parse.

forge.joomlapolis.com/issues/7902