For Ustreaming a Gaikai (or RemotePlay) session, the easiest way is to push the responsibility to the client since the Ustream UI also shows a player video chat window -- assuming the network is robust and the client is powerful enough.
Yes, my expectation is at launch, we will "only" get a subset of what's announced. The OS may not multitask everything pre-emptively and freely. Perhaps like Vita, there will be some restrictions for various reasons.
I don't think ustream will be able to handle multiviewer with remoteplay. Latencies and ustream server requirements are too much unless it's a game that allows for more than one second of input lag.
Gaikai works under a complete different infrastructure and is supposed to optimize for latencies so I cannot comment on.
As a streamer, most streaming sites introduce somewhere between 5~10 seconds of lag. Point to point is the way to go.
I don't expect the remote play upload to be gigantic.
Somewhere along the lines of 5~8Mbps would probably be fine so most people that have decent internet connections in decent areas should be ok.
Many streamers get away with 500kbps ~ 1mbps and are still "watchable".
(however I have seen stuff from ~15 years ago that allowed people to phone in and play a game on broadcast TV with their phone lines and touchtones.)