If there are empty seats on the other flight you don't need to stand by. SDC is a confirmed change. If there's a ticket for sale in your class of service, you can get it. If not, you can stand by. So, yes, you can buy the cheapest flight and try to change to a more convenient (more expensive) time for free. If you are flexible with your travel times, there is a lot of value there.

You are supposed to be on the upgrade list if you SDC. But there have been reports of it not working properly. Also, I'm not completely sure about how it's supposed to work, since complementary upgrades are (I believe) processed in the order that people are added to the list. Normally people are added when their upgrade windows open, which ensures that it's in order by elite status, but, since you'll be being added to the upgrade list right before takeoff, well after the MVPs have already been added at T-48hr, I think you'll pretty much be dead last on the list.