To answer your question: Number 1, we don't know him personally, but DarkStar has been here since 2015; he's quite popular and trusted here on Bitcointalk, just so you know. As far as I know, he rarely manages signature campaigns because he focuses on managing the Chipmixer signature campaign. Number 2, he didn't disappear; he's just taking some rest, as he stated in his signature space. He might want to not get involved in what's happened with Chipmixer.
Regarding Chipmixer's owner and the situation happened to Chipmixer case, this article might help to answer you question:
Germany and U.S. Seize Over $46M Crypto Tied to ChipMixer InvestigationI don't understand why this post gets merits.
Maybe try asking JayJuanGee and NotATether because they're the ones who sent merit to my reply.
Why did Chipmixer choose him for signature campaign when there were many other experienced signature campaign managers?
I already said that might be the reason it is trusted and popular. But if you really want to know the answer, try asking ChipMixer or DarkStar_; they're the only one who know about their deal signature campaign management.
Is he really just taking some rest?
Try to ask him if he really is.
You also said that he is trusted, but right now it looks like you don't trust him because of the way you asked the question. IMO.