2a) Does it map to a setting per journal or per comment?

I think you mean per entry--in my testing, it only respects the warning on the entry. A journal marked 18+ with an unmarked entry will still show entries from that journal in a filter where only unmarked entries were allowed. I feel like this is a bug, but on the otherhand if somebody put a 18+ journal but only allowed NSFW or lower, then they'd never see any posts from that person they put onto their filter, hrm. Will have to bring this up somewhere.