As other mentioned it has singple player Scenarios and Skirmish mode, including two special tower defense/survival modes. We are also building a dedicated campaign now.

BAR does have a series of scenarios for singleplayer / offline play.

Or in multiplayer you can arrange a co-operative game with humans against AI opponents, which often has substantially less flaming involved, especially when playing a "survive against an onslaught of enemies" scenario.

Also the account system of course allows for muting, avoiding-being-paired-with, or fully blocking players. For more egregious behavior a player can be reported to moderators and temporarily / permanently suspended if they break the community code-of-conduct.

Me and my friends just play against the AI. We'd last 2 seconds against real players but we're slow and old RTS gamers and just have a good time stacking up a few AI opponents and smashing them.

No campain but there are stand alone scenerios you can play. There is also a fun swarm defense sort of thing(rapters) if you like tower defense style gameplay.

Yes, it also has a lot of co-op mode and a non-trivial portion of the player base focuses on that instead of PvP/competitive modes.

Yes!