dpt135, not to drain your excitement, but you will NEVER receive a LOA to NAPS, they just don't exist.
A letter of selection is though!
In fact, if an applicant has received a LOA, a letter of Academic Qualification, is 3Q'd, NAPS selection is no longer in play. NAPS selection is generally reserved for those applicants that the Academy would like to consider but do not have the academic qualifications for direct appointment. Hence, if you receive any of the qualifications necessary for direct appointment, then one is no longer eligible for selection to NAPS, you're swimming in a different pool. And a tougher one at that with seats offered limited to about 300. I'm not a numbers guy but I think that's even less odds.
On your other note, it's best to have one's application complete with test score at any level. An incomplete application does not get reviewed. You might as well not submit one. Having said that, the Admission Office recognizes that you can not submit all the required info at the same time, so the application gets built on as time goes by.
Just remember that other completed applications are being reviewed while you are waiting for 'just the right test scores'. Application supplementation with new and improved test scores, awards, letters of recommendation, etc. is expected and anticipated. But still, the application is not reviewed until it's complete (if anyone has contrary information pls share).
My DS submitted his application early and continued to submit new test scores and awards as they were received. The Board continued to review his application in context of the other applications being considered. His NAPS selection letter came 12-26.
Good luck and stay in the hunt!