Ok so on SC, you have a "user name" that you create your account with and can never be changed, but the "profile name" is whatever the receiver has you listed as in their own phone. As a default, when you create your profile you can leave it blank or enter your name. When someone you have friended gets a snap from you, it will show up as the profile name you have entered if you have done that, if not it will show up with your user name. And then on their end, someone you have friended has the additional option of editing your profile name as well, and when they receive a snap from you it will take precedence. So whomever screenshot those pics that LE uses that opened them from Libby is someone that either has her listed as "Liberty German" in their SC contacts, or Libby herself entered her name in her profile as Liberty German, she does seem to use them both.
Twitter: libby1227 (does not appear she was active on this account other than the day she created it in 2014)
Kik: lrlgerman
Snapchat: libbybaby1227
Instagram: Its_A_German_Thing (this account was listed in her twitter profile but no longer exists) my guess is either the family have deleted it or it was an outdated account. Most teens her age use IG quite a bit so I would expect she had an active account at the time of the murders, just not sure it was this one.
Facebook: the family has taken it over as an "in memoriam" page and it uses Liberty now
As for your last question...like I said, if she was actively communicating with someone on SC at the time of the murders and LE does have her phone (not just access to the cloud) they would have been able to see that, just not the actual pics they sent each other, or the chats unless one of them "saved" it. Even if that person suspected LE would see them on her contact list and be suspicious and "deleted" her as a friend, their user name would still show up on her communication list but greyed out (not their profile name, just the user name). BUT the only thing I am not positive about is if someone actually were to have "blocked" Libby after the fact to try and avoid a tie back to her SC, would their user name still show up in grey on her recent communication list...my hunch is no. So if that were the case, that would probably be something they would have needed a warrant from SC to recover, but I do believe all of that is recoverable so LE would surely know that by now, just not right away at the time of the murders. Hope that helps!