In Book 2 (Goethe Institute) they use "Bay Müller". Isn't that correct?
"Hos geldin" doesnt work for this one?
Since you address that person using the word Bey (Mr.) we understand that it's a formal conversation so you need to use the second plural ending (-niz)
I suppose the "Mr." is supposed to signal that you're expected to use formal/respectful (siz) language in this sentence.
"Welcome, Mr. Kaan!"
Translation: Hoş geldiniz, Kaan Bey!
Just being stupid here: Star Trek - "Into darkness."