To get the messages from that board to not appear just in that component would unfortunately require a customization.  The customization typically involves creating a custom setting to exclude the board from the landing page activity feed, and then in the activity feed component we have to add logic to get a list of boards that DON'T have that setting set, and get the messages from those.  At the very least adding the custom setting would likely require a Professional Services engagement to create the setting and make it available in the API.  ... View more

As of 22.3 the API (which is used for that activity feed) will now respect the admin setting "hide messages in this board from lists". Admin > Community Structure > (Hover over a board) > Edit Properties This will keep the messages from that board from appearing in message lists outside that board (unless explicitly queried for). They will still appear in places like the search. ... View more

How do you know the attachment is there when it isn't showing? Have you tried inspecting the post with the browser dev tools to see if the markup for the attachment is there? Having attachments only appear on some posts but not others in the same board is very unusual. ... View more

Hiding ("Hide from lists and menus" setting) a board or category, does only that: hides it from showing up in OOB lists and menus. It does not prevent someone from getting to it via direct link, or it appearing in API results, and obviously, from RSS feeds. If you do not want the board to be publicly accessible, you have to restrict it using permissions. ... View more

Without authentication, you'll only be able to get public content. So if that message UID is in a private area that would otherwise require authentication, you won't be able to get it from the API without authentication. ... View more