Hi,
I am a first year med student and it occured to me today to see how all the applicants are doing. I know I appreciated help from med students when I was in your shoes.
One thing I am sure of is that letters of intent are most effective when you can say, "I have been accepted to a medical school (don't name the school though) but I prefer yours because xyz." Obviously you bring out your best ass-kiss writing skills and make it specific to the school. See if they have a mission statement and use key points in that (rephrased) to say what you love about the school. Example: I said I loved the school of my choice because it had a progressive vision with a forward-looking attitude or some crap like that.
It is harder to be taken seriously when you have no admissions under your belt. Adcoms have very little clue about you really and are always afraid the adcom at another school knows more than them. If someone else let you in they think you are wonderful and they missed some amazing quality and if you haven't beed accepted elsewhere they think the opposite. That is the advantage of applying to schools you would NEVER want to go to but think you have a chance of getting in to - it gives you leverage at the schools you like. It is still worth writing letters (every week when spring rolls around) and calling too (check the policy on calls though - some schools don't allow them).
Never give up. Waitlists are a call to battle!
This is all very depressing for people with no admissions I know - but it was my experience. I will also tell you what I tell everyone. If you don't get in the first time TRY AGAIN. It is a long complicated process and (I know) it is tons easier the second time.
I also think letters of intent are far more powerful than letters of interest. It is hard to get around the fact that you are not expressing intent. I sat down once to write 3 letters of interest and they all came out sounding watered down. I sucked it up, picked a school, and wrote a letter of intent. Worked like a charm. But before you pick a school make sure the waiting list there still has turnover. WUStL had 0 turnover really early in the year 2 years ago - almost no waitlisters got in - the admissions officers will tell you things like that if you ask general questions. Not "what are my chances" - though that would be nice!
To answer the other question - definitely send letters to the dean of admissions. If they don't have one -> head dean. The office staff knows enough to get it to the right place.
I know this is getting long but my other peice of advice is to give out your cell # and then carry the thing charged at all times. If I hadn't picked up the phone on the 2nd ring when I got the call that I was moving off the wait list they would have moved on to the next person. No joke.
Believe it or not it is still early in the process so try not to sweat it too much. Congrats on getting this far. Good luck. Hope I didn't sound harsh!