there are two cases here:
1. the O/A cycle has been completed, and suddenly packets with an unannounced SSRC is received at the IP:port assigned to an existing peer connection. My understanding is that you should drop the packets.
2. The offer has been sent, but the caller hasn't received the answer yet. suddenly packet start flowing to the caller. This is called "Early media" and was added to the webrtc specs in november 2015. I do not know what the browser support is today. You can create a transceiver to handle early media. There are some subtleties about encryption and codec settings, but you can refer to the specs for the details.
I do not think webrtc ever require you to MIX media. You can do it at the application level, or within a media server if you want though.