Es,
Amazing is most likely correct. You are correct also in that they probably viewed your DA letter as a "family" matter and allowed your father to destroy it, in many situations elders families are given more leniency. That is why they need to meet with you on this new charge. The truth is, even though the DA letter was destroyed, it will be held against you as a sign that you are not repentant because they suspect you are not only commiting a sin but also a doubter.
Elders and CO's prefer to see DFings instead of DAing's in my opinion. Anytime someone DA's themself they are suspected apostates since it is a wholly voluntary action unlike DFing.
My advice, just dont bother to meet with them period. If you dont beleive in it anymore then forget it. Not worth the hassle.