I was wondering if Stauffenburg could've been a Lame Duck given his qualifications

"In June 1944, Stauffenburg was promoted to Colonel and appointed Chief of Staff to Home Army Commander General Friedrich Fromm. This gave him direct access to Hitlers briefing sessions."

By definition a lame duck is: An elected official who is meeting the end of his or her tenure... choosing not to contnue his or her tenure...

So the question becomes what constitutes an Elected Official?

An Elected Official: Is a person who is an official by virtue of an election

... So the argument could be made that he was elected (appointed Chief of Staff), since there are multiple sorts of elections by the manner in which they can be carried out ; the sort of election (process) in which Stauffenburg may have been designated under could be something like what we here in the u.s. government call a By-Election or perhaps Co-option.

And so this is where the confusion takes place, given "the manner" in which he acquired his position in office.

I wouldn't call him lame duck. Considering the confusion with so much of what went on then, I don't think we can apply today's standards and definitions to what went on back then in the Nazi regime.

... just my opinion...

I don't agree with your definition of lame duck - I don't believe the "lame duck" is limited to only elected positions. If one is appointed to a post that has a definite end time, and it is known that the person will not be re-appointed at the comnclusion of the term, I would call that being a lame duck. For example, at the end of the Bush Presidency Condoleeza Rice could have been considered to be a lame duck Secretary of State, even though Secretary of State is not an elected position.