My opinion is that you don't need to really do any extra critical reading for the reading comprehension section on the DAT. The DAT just tests your ability to locate details within an article. That's all it is - nothing more. Albeit, journal articles have lots of details, but it is on a whole other level (way more difficult) when you are comparing it to the reading you will see on the DAT. If you've picked up a Biology book before and taken any number of upper division science classes, you will be more than prepared for the type of material you will see on the test. All you need to be doing on test day is :
1. skim passage --> 2. look at question --> 3. scroll back up to find answer --> 4. answer question (Repeat steps 2-4)
Does that sound like something that drudging through journals will help you succeed on? No. Just be able to get an idea of where details are kind of located (you can try paraphrasing if that helps) so that when you get to the questions, you can quickly go back and find the answer. Honestly, the time that you could waste reading journals could be better spent studying the other sections. Not only that, but you can spend half of that money for a subscription and go to Barnes and Noble and buy the Kaplan DAT book ($50) and memorize it front to back. I did that for the DAT and I did fine on the test.
Also, about reading journal articles for interviews...why would you do that? I admit that I didn't go to that many interviews (I only went to my top choice and second choice schools), but at no point in the interview did it turn into a free forum where we could talk about whatever we wanted. The school will use the time to impress you with their school and to get to know YOU as a person (from what they couldn't pick up from your application). I'm sure letting them know how serious you are about the field is nice and all when you start busting out journal topics, but haven't you already shown your seriousness/commitment about dentistry as a career choice by taking your pre-reqs, applying, and taking the DAT?
So my opinion in regards to journal reading is that it is completely unnecessary. I teach the DAT for Kaplan and I promise you that reading journals won't help you that much on the test.
Also, to answer your question about your essay, just write about yourself. Let the application committee find out who you are and why you chose dentistry. The style of my essay is kind of a hybrid between the college essay (cleverness/fun to read) and a personal statement (getting serious points across/informative). When you are writing the essay, you can write it in a Word document and paste it in later. You can also do parts at a time on the website itself if you want to as well (but since the new application isn't available for another few months, just do it on your computer for now).