B)What levels of criteria do you think will spur LOA letter?
I'm first in my class of 473 & 2210 SAT,National Merit Semi-finalist ,
B) is almost an unanswerable question. And really not the best way to look at it.
Just a dad with a son who just went through this... but my suggestion would be:
1) With a 2200+ SAT that's in the range for academic type LOA's based on briefings last year at SLS and info sessions. (really 700+/700+ reading/math) But you'll either need athletic experience or submit a strong CFA so it's clear you'll be able to compete physically. And matching leadership experience. So in your shoes I'd hit the CFA hard, and work on getting a solid one in ASAP. (note: it might be to your disadvantage to turn in a weak one early)
2) Get your medical clear as soon as you can to remove that as a possible question. (and you need to do it anyway)
3) Meet with your MALO or similar to review your EC's, athletics, etc to look for holes/gaps/opportunities.
As one helpful field person put it: "You want to make it easy for them to assign you points". That means removing any unknowns (CFA performance, etc). Some of this is mitigating potential stereotypes that might go with strong academics.
4) Work on completing your file..... don't wait to see how LOA's play out
5) Get ahead of your nom process. This means downloading the forms, talking to the staffers, see what they look for. In very competitive states they have their own scoring process.
And as several have said, and will say again, LOA's are the exception rather than the rule. What applied last year may not apply this year. You cannot and should not count on them. (though you can hope and try!)
Given the above, the best way to improve your LOA chances is to improve your competitiveness overall. And keep trying to improve!