+1 ^^^^^
I wouldn't wait until right up until 4/12, but if it was my DS and he hadn't received the BFE for USNA by 4/10, I would advise him to expect the TWE from USNA and make decisions from there. If USMA is truly your 2nd choice, and your priority is serving rather than the branch of service, my advice would be to accept it. If you do get a BFE after accepting, contact WP admissions and inform them you are going to decline since your first academy choice finally came through. I'm sure in this environment, they will understand your reversal (even to Navy!) and there are plenty of other qualified candidates who will be thrilled with a call in May or June asking if they would still like an appointment.
I think in the past they have given until May 1st to accept/reject but in this budgetary climate they added the 45 day decision period since there were no rolling admissions and they still want to fill the class.
In our case the choices are reversed and when DS received the WP BFE he accepted the next day. If he does get a BFE from USNA, he will respectfully decline it so that someone else can be offered. We know he did not win the USNA MOC slate so a USNA appointment could only come from the NWL.
BTW, I am not military and have no teeth in the game but would rather have seen DS go Navy as my impression is that it has a broader application of technology and his love is Engineering. But his heart is in the Army and I understand that. Point being - you have to go with where your heart is...