If it's a LOI or activity update or something I know they will definitely put into my file, I send it in as a pdf. I go extremely formal on these, with the 2 addresses on them, date, etc. etc. The subject line I use is to the point (i.e. "Activities Update" or "Update Letter"). For the body of the email I say something like:
Hello,
Attached you will find a letter detailing my activities following the submission of my primary application.
All the best,
aDreamer
(that's almost verbatim).
If I'm sending in something that probably won't go on my file (thank-you, a question), then I'll just write a slightly less formal email with everything there in the body.
With your other question about writing to David Owen in particular, I think you're good with the direction you're going. I did a similar thing with Joni after a few months had past since my interview/thank-you and I wanted to reach out to her in particular since she would likely have been the most familiar with my file (wanted to see if there was anything else I could address/clear up). I just opened with:
"Hello Mrs. Krapec,
It's been a few months since I interviewed with you at Pritzker. Since then, I have learned that I am a continued candidate. Firstly, I should say that I am absolutely ecstatic to continue to be under consideration. Secondly, in addition to conveying my sincere interest, I am hoping to provide as much information as possible to the committee as they move forward in the decision making process blah blah..."
That's how I've been doing it anyway. Anyone have other methods/thoughts?
Hope that helps! We're all in this together!
Disclaimer: I used to work at my school's career advising center so I'm probably way more neurotic than your average applicant!