SACD a 5.1 medium?

All SACDs have a stereo track. Most are superior to their CD equivalents although that is dependant on the specifics of mastering.

That's reassuring. It leads me to another question. Say a two-channel master is remixed into multiple channels for SACD. In a traditional two-channel setup, all the information from the master would be sent to the two channels. In the remix, all that information goes into multiple channels. If one were to replay that remixed SACD through the front channels only, would some information be lost?

Sure but why would you? I repeat: All SACDs have a stereo track. That means that on a MCH SACD, you would play the stereo track if you do not have a MCH system.

Besides, who needs a stereo master remixed into 5.1 fictitious channels?

Amen. I was worried that remixing would degrade the original stereo image. But if the SACD discs are encoded with two-channel tracks as well, that's not a problem.

One more question. How does SACD improve the sound? In other words, what does the SACD process achieve that makes it superior to redbook? I'm assuming there has to be some improvement; otherwise, there'd be no need for the format.

(Not looking for a flame war here, just information.)

Use google to find the technical differences. Subjective comments can also be found although you may get some right here.