Even after I got replies that were similar, if these were letters sent back and forth as part of a job application and no offer had been made, I stuck to using Mr./Ms./Mrs. lastname.
My personal opinion is that should be the way forward, unless they specifically tell you to refer to them by what they want, and at that point, I would switch to whatever they suggested. If they never suggest a change but sign their emails with only their first name or anything else, I still go with the full name. If I notice in their signature something important, like a different last name (hyphenated?) or a Dr. or some such qualifier to add before or after, then I WOULD add that in to show that I am paying attention.
Until a job offer, however, I personally don't think there's room for informal salutations until it is requested, or in very rare cases where you are already very close with the individual... but at that point the informality should already be obvious (or it would not be appropriate then, either).
Some may find no offense if they do sign their emails informally, as they may recognize that you are merely using the details available and attempting to follow suit. You may be able to get away with it, or they may even prefer to see that without them having to prompt you to shift to informal conversation. That said, I cannot see how there would be any fault for an applicant retaining the formal approach throughout all back and forth, at least prior to official offers.
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