1.) Why can't you buffer on the client side for WebRTC? That sounds like a client issue (what library were you using?) not the protocol.
2.) I use the same tactic as HLS. Generate your video with a reasonable (~2 seconds) keyframe interval. When a new client connects start sending at the keyframe.
2.) I use the same tactic as HLS. Generate your video with a reasonable (~2 seconds) keyframe interval. When a new client connects start sending at the keyframe.