2. Would having someone as my default trust makes his trusts visible to my account? (this seems pretty logical)
3. What does Depth mean? Beside the max of 4, there's number 2 -- DT 1 and DT 2 right? if i changed it what would happen?
1- Yes if you tag an account with a negative trust then you will add to it a negative score.
For example i have 0: -0 / +0, if you add a negative trust to me, you will see -2: -1 / +0 and when you enter to the trust page, you will find your comment on the "Trusted feedback", that's because in "your" setting "you" are part of the default trust. Now for me or for any other member who didn't add you in "our" default trust, nothing will change, we will keep seeing invoking having 0: -0 / +0 but if we look closely to the "Untrusted feedback" and we press "Show ratings" we will find your feedback.
2- If you add someone who isn't part of depth 1 or depth 2 to your default trust, you will see that some Bitcointalkers that had previously 0 trust with -2 and the opposite things for the others which depends from the type of the feedback he left : Negative or positive.
Now if you exclude (~ then name) someone from depth 1 or 2 from your trust settings, his feedbacks will not appear for you anymore in the Trusted feedback but instead in the Untrusted feedback and thus means you will see his feedbacks deducted from the trust rating of users.
3- theymos for example is depth1, think about it like a grandfather, he trusts his children and add them to his circle so they become a depth2 default trust members. Then those children trusted respectively their children also so they become depth3 and so on, so the next generation become depth4.
Press the "Hierarchical view" at the end of the page to have an idea about this.
You can trust only depth1 (theymos, HostFat, dooglus, Maged, dserrano5, OgNasty, Tomatocage,
SaltySpitoon, philipma1957, Cyrus, Blazed, hilariousandco, OldScammerTag) or depth1+2 or 1+2+3 or the four.
Now you can edit your trust settings as you wish and you can personalise it by adding or excluding any member you like but keep in mind that the others will always see depth2 as the default trust (apart from some members who edited it of course).
I hope I was clear in my explanation