A couple points. The fact that he no longer has the memo, but he had it after being fired, is a sign that he "wiped" his computer. At least one memo was created on a computer, and everybody knows that "sharing" digital material such as documents, notes and e-mails is fundamentally an act of copying. The original exists until it is deliberately erased.
Comey and Mueller worked together on evidence destruction, or Comey did evidence destruction on his own. I draw Mueller in based on Comey's testinony that Comey no longer has the memos, he gave them to Mueller.
As for classification, I totally agree. Material created that memorializes a private conversation with the president is presumed classified. It is up to Comey to demonstrate that it is not, and his conclusory assertion doesn't cut it. The FBI didn;t publicly admit until recently that it had Flynn under investigation, that was a leak. Comey's memo CONFIRMS that Flynn was under investigation.
Who appointed Mueller as Special Counsel? Sessions? Whomever did it had better un-appoint him ASAP. He’s a wolf in sheep’s clothing and we can’t afford to deal with any more chaos. There’s no crime; there should be no investigation/”matter”.
All private conversations ..even when you make notes later ..are not considered classified just by virtue that it involves Trump is my understanding...but I have no regs..or case law to point to.