I believe Orthodoxy does deny the reformation its validity. I can’t deny positive elements to it on the whole however, ie challenging the authority of the Pope and challenging the corruption of the church of the time but that doesn’t justify that enterprise.
The Lutheran and Orthodox dialogue in the 16th century is probably what best reflects the Orthodox understanding of the reformation, specifically the Lutheran reformation.
IIRC, those original dialogues consisted of several misunderstandings and mistranslations between Greek, German, and Latin. But they did end cordially, with Jeremias II referring to future communications as “from a friend,” even if he did not consider the Lutherans to be Orthodox. Why wouldn’t one remember and mark the shared history of a friend?
Separately, how the ELCA and the largely-unorthodox LWF choose to commemorate (whether through festive celebration or solemn remembrance) has no bearing on how much the Orthodox can participate or in what spirit.
The rest of us more “Lutheran” Lutherans will commemorate the reformation as the solemn necessity we believe it to have been. This is not dissimilar to how one might commemorate WWII, its tremendous loss of life, yet incredible victory of freedom. It might’ve been Krauth or Sasse who said something to the effect of, “The good Lutheran must rise each morning and justify his division from Rome.” This,
as our confessions state, is a grave matter. Lutherans, at least the sort that value their Confessions, will treat it as such.
O come desire of nations bind,
And join in one the hearts of all mankind.
Bid us our sad divisions cease.
And be Thyself out Prince of Peace.