Student or Learner
"Do you happen to know with who Anna left for London?"
"Do you happen to know who Anna left for London with?"
Is there any tendency where to put "with" in this context?
"Do you happen to know on what floor their appartment is?"
"Do you happen to know what floor their appartment is on?"
"Who did you give it to?" is more common than:
"To whom did you give it", which is more common than both:
"To who did you give it?" and "Whom did you give it to?"
(In my experience)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Immediately after a preposition? Yes.