I am a current high school teacher. I think your proposed course of action -- passing up SLE because of the conflicts of (a) the SAT II subject tests; and (b) Boys' State -- does make sense within the context of vigorously pursuing an appointment to West Point. I don't think you'll get any "bump" from the fact that you got the SLE offer, though. I would also make sure that in contacts with admissions you make sure to explain the conflict. You should also be prepared to explain yourself should the question arise in interviews with, say, your Congressperson's nominations committee.
As many have pointed out, SLE is, first and foremost, an opportunity for USMA to raise awareness about West Point with individuals and communities. Quite a few SLE attendees don't get appointments, and conversely many who didn't get a slot at SLE ultimately get appointments to West Point. As others have also pointed out, attending Boys' State adds more points to your candidate score. Now, if you are still really on the fence about whether even to apply to West Point, or whether the military lifestyle is for you, that might be a reason to pick SLE over Boys' State. But in terms of improving admissions chances to West Point, Boys' State is the clear pick.
Now, moving onto the SAT II subject tests. We always tell candidates that they need to have a "Plan B" and not put all their eggs in the Service Academy basket. Most of the top colleges -- from the Ivy League to Stanford to Rice -- require two or more SAT II subject tests. It makes sense that you want to take the "main" SAT in May, and that leaves June as the possibility for taking the SAT subject tests for courses you have taken junior year (for example, U.S. History).
So, this was a long-winded way of encouraging you not to second guess your decision-making, which is rational and not inconsistent with a strong desire to go to West Point.