For some of the things you all are talking about, I can at least relay my/my class' experiences.
1. Switching services. We have 4 people switching to the Navy and 2 to the Army. The 2 to the Army are doing so because they married Army classmates, as well 2 of the Navy. The third switching to Navy is married to a non-USUHS affiliated Naval officer, and the fourth has a high ranking Naval officer parent who, I believe, helped her switch (either just because of the high rank or because they actually helped push paper work through). It can be done. If you end up in the situation of trying to switch - do so as early as possible. At least a couple started the process a year ago and none of them have actually been switched yet - which means, none of them have orders for internship yet.
2. You don't have to drop other waitlists (unless they changed something since I was an applicant). If you are accepted to another school you have a certain amount of time to decide between the 2 schools (2 weeks, I think).
3. For the Navy, there is no complete physical at ODS, but we do have a swim test (it is the Navy after all). There were several people who got tattoos while at ODS; from what I remember it was on a whim, so they didn't tell anyone beforehand. But I'm sure they told their senior chiefs after and maybe had to do something to update records - I'm not sure. Being overly cautious is probably your best bet.
4. You can get accepted into the summer, and once you sign your oath and are commissioned, you are in. It is a contract between you and the government. I had a classmate who was accepted 1 WEEK before school started. Again for the Navy (sorry I don't know about other services), if you're accepted beyond ODS, or they mess up your orders, you can do an abridged version of ODS at USUHS. For the people accepted too late for that, you can do ODS during one of your 4th year electives or before you start internship. There's a woman in my upcoming intern class who will be late to our orientation because she has to do ODS - she's HPSP, so I'm not sure what her story is, but it's an example of what you can do if you're accepted too late to go to ODS.