Was Chris Mannix actually the new sheriff of Red Rock?

There are many unanswered questions in this movie. But you knw what? It doesn't matter. Mannix got what he wanted and it's like his request was potentially harmful to anyone. He just wanted a ride to get in safety, so he manipulated John Ruth to accept him in. Notice that Ruth never fell for it, and still accepted his request "just in case it was true". Telling people he's the new sherrif gave him a sense of power, a little bit like Warren's letter (which was revealed to be fake). So whatever he was or wasn't doesn't change anything since they all died anyway at the end.

I feel like he really was the new sheriff or it's extremely random he knew about the hanging of Lance Lawson.

Also it seems at the very end when him and Warren were hanging Daisy he does a little speech about being the sheriff; there's no point of continuing the charade at that point..