Sorry about the delay, I'm trying to figure out your problem here...it looks like in your next to last screenshot there some of the attachments are not attached to posts, and therefore there's no checkbox...is that correct? Can you try attaching one of the detached uploads to this thread in a post and see if it makes it selectable again?

If you go to reply to this thread and click the Go Advanced button, then click manage attachments, do the attachments you wish to remove still appear in the asset manager?

If they do, there should be a question mark that appears in the top right of the thumbnail of each one when you hover over it that says asset usage...see if the asset usage lists anything and let me know.