I doubt WT will ever give details on how they select for Treg, but based on things they have said in the past, they want a wide variety of users. "Random" is probably an exaggeration, but to some extent it is what they need. But within that randomness, they would seem to pick some based on a particular use case and they probably select from people they think are thoughtful in what they say. And probably some based on different countries (thus different layouts). So, random in some ways but not totally random.
But any test group has to be limited simply to make dealing with the reports within reason. As early tester's reported issues are dealt with, new ones may be added - simply because the early testers probably have little new stuff to report and that makes it possible to absorb new testers efficiently.
You don't only get $19 free multi-map. There is also a free nano charger and nano stand which total another $14, so a total of $33. That's a pretty big chunk of the price of the TB itself! And that doesn't count the free gift. Of course, only testers know anything about it and we only know what the "top" gift is (WT has said the sooner you order, the better the gift). We can't tell anyone under the NDA, but it is really really good. But we don't know how the gift changes for later orders and where the dividing lines are. So for orders now, it may be extremely minor. Or not.
Which brings us to the issued about "announcements soon". Which is a legitimate point.
Not sure what to make of WT's post a couple hours ago, saying more features and higher price. It could mean, after not saying a lot for so long, that we are getting close to shipping. Or it could just be telling us some of these changes, but not mean they are close.