You'll get a few opinions on some of your questions, here are mine:
1. Sonically, both of the new formats are in my view better than redbook CD, and I could live happily with either of them. They are a bit different from each other, SACD to me sounding a bit smoother overall, but both offer excellent sound.
2. Multichannel SACDs have a two channel mix on them, so they'll play on two channel players. It will sound as good as a normal two-channel version of the same disc; the question is how well the engineer and producer did the two-channel mix from the multichannel mix.
3. Machines will vary as to their default settings, I believe, but their displays will show whether the CD or SACD layer is being played. On my SCD-777ES, there is a button to push, and it will stay on SACD if I don't listen to a CD; once it goes to a CD, I have to re-set it by pushing a button to get the SACD layer.
4. You'll get plenty of opinions here, and you can check the archives. I think they are worth the extra money, but CD has made a lot of progress, and the improvements over CD are subtle enough that if you have been brought up listening only to CD you might feel that SACD/DVD-A are too soft-sounding and not as clean or "sharp" sounding, and not worth the extra money. If, like me, you were raised on analog and vinyl, they are closer to the real thing than CD and have a lot of the naturalness we associate with vinyl and are worth every penny. Both formats, unfortunately, have been unsuccessful in the marketplace as a replacement for CD. If you like classical, SACD has a very good catalog, and a lot of smaller labels in the classical area have been putting out new releases on SACD, so there is hope that SACD will continue as a niche product at least with that genre of music; if you're into pop, I'd say stick with CD.