Click the + icon next to Channels in the sidebar, or use ⌘K and type 'create channel'.
Yes — hover any message you sent and click the pencil icon to edit inline.
Open the channel, click the channel name to open details, and toggle 'Mute notifications'.
⌘K opens the command palette which doubles as global search.
Click the workspace icon at the top of the global sidebar to open the switcher.
Relay is dark-first. Light and System modes are available in Settings → Appearance.