I am a fellow ROTC HPSP student so I will give you my opinion.
1) As far as getting the scholarship, it may give you a slight advantage, but the down side to being ROTC is that you cannot take advantage of any retention bonuses until you have fulfilled your initial commitment, which for me is 8 years (4 ROTC, 4 HPSP). Things may have changed since I applied 4 years ago, but at the time if you had a 3.7 overall GPA and a 19 on the DAT, you were automatically accepted for the HPSP as long as you got the paperwork in on time.
2) You do need a letter of acceptance to get the scholarship. LDAC doesn't help you in any way, but I'm sure it would hurt you if you totally tank it.
3) I used Kaplan and I thought it was totally worth the money, but other people do just fine studying on their own.
In summary, I would shoot for a 3.7, and better than a 19 on the DAT. Plan on getting all of your paper work done as early as possible, and don't suck at LDAC.
-Also in my case (and presumably yours), if I didn't get accepted into dental school immediately after graduation, I would have been commissioned as a 2nd LT into the Chemical Corps (not that there is anything wrong with that, just not how I would want to spend my time in the Army). The military doesn't just wait around for you to get in. They will find something for you to do. It turned out to be huge overkill, but I applied to 17 schools just to make sure I got in the first time.