Would you expect the US to invite the British and Russians to commemorations of the liberation of the Philippines on the same grounds?
What is the exact harm in it? I'm always highly impressed when foreigners come to Australian war ceremonies of their own accord.
Yet no one would think it odd or unusual if a Russian veteran or someone with a relative involved chose to go personally for whatever reason. I'm sure they'd be most welcome and hopefully treated with as much respect as any campaign vet or other attendee. I would think it odd if our PM was invited to say commemoration of the Battle of Kursk, despite that being of huge significance. I'd expect a good few Russian veterans or relatives might think it odd too.
Commemorations are for the veterans and relatives of those that took part, and to honour the participants. Which tends to make it expected to have official presence for the nationalities that took part in each campaign, the former commanders, and the descendants of those who took part, or who lost a parent or grandparent. For many attending it's a deeply personal act. I'm uncomfortable, to say the least, with commemoration as national or political PR.
In the early 1930s the Russians were evaluating a lot of military stuff and basically settled on "well, given these advantages it makes the most sense to not try too hard to specifically prevent our territory from being invaded, just let it happen and then with the help of a winter unlike anything in most of Europe slowly crush them on our home turf with our manpower advantage." And that's exactly how WW2 turned out. Sure they botched the start of it and lost more men and territory than they were expecting but they basically did the obvious thing. Hitler just thought he would be able to take all the strategic assets in the West of Russia, capture Moscow and force surrender before that happened. He was wrong. Just like everyone else who's invaded Russia.
Union's army was going domino up until Moscow, and it was defended with last reserves of reserves from the whole theatre.
Germans on other hand, were beyond simply overconfident. I watch one documentary after another showing German army having fiestas, and things along the line "Hitler ordered general A to stop and hold flag raising ceremony in city B"