I hope you put this behind you by now. I don't care if some elders believe me or not. I know you want the girl and people involved to believe you, but even the polygraph might not change that.
I have to imagine it's someone who was sure you would get the blame who wrote the letter. That would make more sense than many other possibilities. Could be the Navy guy or someone who has you on their hate list. Or it could be another lady, jealous and would've like to marry an elder, who can't believe she gets to live such a past and still be married to an elder, happily ever after.