Those on DT2 can select members as well, and they're reviews will appear as trusted feedback as well. If you've left all your defaults set, that's how it appear from your perspective.
That's not accurate. Only feedback from DT1 and DT2 members show up as "trusted" by default. The users who are added to the trust lists of DT2 members effectively become DT3, and their feedback would only show up if you set your trust list depth to 3. The entity "Default Trust" is entered at level 0, DT1s are level 1, and DT2s are level 2, so with a depth of 2 you see 2 levels of trust.
Note this is different than if you have a custom trust list. If you have a custom trust list, then anyone you add to it becomes level 0, their trust lists becomes your level 1, and your level 1s' trust lists become your level 2. So with a depth of 2 you do in fact see 3 levels of trust.
On your trust settings page you can change the depth to 1, which will mean you'll only see DT1 and DT2 reviews as trusted.
As above. If using default trust then you need a depth of 2 to see DT1 and DT2. If using a custom trust list then a depth of 1 would achieve the same thing.