Translation:Hoş geldiniz, hanımefendi!
No, the moment you asid "hanımefendi," you're in "siz"-terms with the person.
Hanım is usually used as an honorific; like Selcen Hanım from a previous question. I'm not 100% sure, but using it as a noun for someone you don't know is rude. Hence the polite "hanımefendi"
Can't Hoşgeldiniz be written as one word like here? I think I have seen it quite often and my dictionary accepts it.
Both forms are abundant actually. I'll make sure both spellings are accepted on Duolingo.
the correct spelling is with a space according to TDK; we accept it without the space, with a typo message of course