So, I called -- again -- and spoke with someone knowledgeable in My Magic+ IT department. She confirmed some things I already posted, and added a few more. Here's what I found out:
The first emails to eligible guests went out at about 3:00pm today. They may be sending them out in batches, but testers will most likely be contacted today or tomorrow. (She also said they were supposed to go out yesterday but didn't for some reason. Maybe that's why the site was live already this morning.)
Because IT couldn't restrict access to only potential testers, anyone whose dates and resorts fit the testing parameters is going to be able to access the system and customize bands/select FP+. However, this does not mean you will actually be a tester. If you don't get the email, you're not part of the test.
Just because you're at the right resort at the right time doesn't automatically mean you'll be asked to test. Only some people at each resort will be testing.
If you link your tickets so they become associated with a Magic Band, but you're not a tester, those tickets are -- to quote the CM -- "lost in space somewhere." Meaning, they're likely invalidated and you'll probably have to work it out with Customer Relations. So, make sure you have hard copies of your tickets with you if you've linked them.