Another day, another CM, another explanation! This one answered the phone "My Magic+ Testing," so it got my hopes up! Here's what he said:
According to the CM, and as PP noted, the emails to potential testers are going out 17 days prior to arrival. Magic Bands will be sent to your home no later than 3 days prior to arrival. However, it's the arrival date at your qualifying resort that they go by, so if you're staying at another resort prior to your MM+-eligible stay, the system doesn't recognize it. They can expedite your MB shipment, supposedly. (I feel like we should be typing in "supposedly" after every sentence at this point!)
Stays at non-testing Disney resorts (aka anything other than AK, CR, YC, and POP) prior to your arrival at a testing resort are not eligible for FP+ reservations. However, if you are checking out and moving to another Disney resort, you can make FP+ reservations for that stay as well. (Supposedly.

)
Also, he said that it's likely -- but not certain -- that we'll be able to access "legacy FP's" in addition to FP+ during this test. (Supposedly.

)
Funny note: Yesterday, one of the CM's I talked to told me something that she thought would happen but wasn't entirely sure about. She said, "Don't quote me on this. I don't want to see 'Becky in IT told me such-and-such' on the DISboards!" To which I replied, " Yeah, you have to be careful of those DISboards!" And, very softly, she responded, "You have no idea."