1. Mentioning everyone in a group is only feasible for "local" mentions in highly controlled types of groups - like a group messenger, where notification may be needed to override settings for those that muted notifications for a group. For example in a group team chat it is OK, because participant may use discretion and only @everyone  when the post is important. If you give this tool for anyone, anywhere the number of mention-notifications would be massive and most would simply turn off such alerts, making these mentions useless. Slack has @everyone mentions for its channels, for example - it works because teams can agree on internal rules preventing abuse of this feature, plus Slack will warn every time that such mention can be disruptive. So, more consideration is needed on this one.

2. For a post to appear in a #channel you just need to #tag it with that channel name. We will add an auto-suggester for #tags to recommend the most popular or trending ones.