Re: Relative clauses
If she is dead, no on else will ever see her alive again.
If someone is missing (and not assumed dead) you can say "Were you the last person to have seen her?" or "Were you the last person to see her before she disappeared?" or "Were you the last person who saw her before she disappeared?"
But no one else will ever see her alive again, so any tense with a present aspect won't work.
(If she was murdered, I hope this person was not the last person - the murderer was.)
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.