WebRTC-SVC: "S" modes and simulcast using a single SSRC

In early drafts of the WebRTC-SVC Extension, "S" modes were included in the mode table but those modes were subsequentliy removed, under the assumption that each simulcast stream would be sent on a separate SSRC.

Recently the Issue has come up again, as part of the design of the AV1 RTP payload, since the AV1 bitstream specification supports simulcast:
https://github.com/AOMediaCodec/av1-rtp-spec/issues/51

Specifically, the following WebRTC-related questions have been asked:

  1.  What are the implications of enabling "S" modes for the WebRTC API? For example, would adding "S" modes back violate the requirement that setParameters() never set "negotiationneeded"?
  2.  Would enabling "S" modes result in SDP changes?  For example, would an Offer with "S" modes be different from one where simulcast is sent on separate SSRCs, but the SSRCs are not signaled in the SDP? If so, how?
  3.   In an SDP Simulcast Offer where only RIDs are present and not SSRCs, how does the Answerer know whether the intent is to send simulcast on separate SSRCs or not?
  4.  For a conference server that supports RIDs,  how much difference does it make whether simulcast is sent on multiple SSRCs or a single one?  For example, RIDs would be present in each packet, but instead of RTCP messages for each SSRC, there would only be a single SSRC.