Well, of course, both time and difference of jurisdiction can affect the answer, but look again at the statement quoted above:

Michigan law imposes an affirmative obligation to retreat upon a nonagressor only in one narrow set of circumstances: A participant in voluntary mutual combat will not be justified in taking the life of another until he is deemed to have retreated as far as safely possible

It seems to me that your robbed storekeeper was attacked without provocation rather than a voluntary participant in a mutual affray. Under the usual tort standards, he had no duty to retreat from the confrontation.

People threaten to sue all the time. People file complaints all the time. But that does not mean that they have viable claims which will survive judicial review.