2. Does this text imply that an RTP receiver MUST be prepared to receive
multiple multiplexed streams on a single session? I expect most
implementations today using unicast are not prepared for that (although
I believe they should be)
No, it would be inappropriate to make that a MUST. We do say, in
Section 6:
RTP application designers SHOULD avoid mechanisms that can only
work in unicast mode and will not scale to larger numbers.
I have a hard time seeing how this implies that receiving multiple
streams on a single session is not a MUST. Indeed, receiving many
streams on a single session is how multicast works, so one would imagine
that the direct translation of this to unicast, receiving multiple
streams on a single port for multi-unicast, would be the recommended
approach.