Is it reasonable to say that Napoleon's campaign was a failure without him having suffered a military defeat?

Depends on how kindly you will treat later stages of the campaign. Some historians consider Krasny and Berezina to be Napoleon's defeats, some consider them to be victories or defeats in symbolic sense. French troops not under direct command of Napoleon did suffer several indisputable defeats though - like battles of Mir, Tarutino, Polotsk. Regardless of number of Russian victories, it is usually considered that none were truly decisive as they never managed to destroy large numbers of French troops in direct engagements - majority of losses suffered by The Great Army were inflicted by nature and difficult logistical circumstances.

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How many soldiers did he lose (out of how many)?

Numbers of French and allied troups brought at the beginning of the campaign are usually perceieved to be around 700,000, of which majority was killed in action, died of wounds, was captured or became deserters. Although losses in personnel were enourmous (usual numbers of survivors reach around 100,000-120,000 strong), many experienced officers managed to escape Russian encirclement at Berezina together with Napoleon (several thousands) and they proved to be extremely useful in coming years of war (used to train new recruits to replace the old army). Several authors like Alexander Zhmodikov consider initial French troops to be somewhat smaller in numbers, around 600,000 or less.

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The same question about the Russians

Russian troops at the beginning of the campaign were much smaller than The Great Army, around 200,000 strong. They managed to escape advancing Napoleon's forces without suffering huge losses and later retaliate with much bigger numbers (800,000-900,000) which made Napoleon slowly retreat back to the border. Their total losses were much likely somewhat lower as they didn't have as many deserters. They were still quite high.

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Which nationalities were most represented in his army?

Majority of the Greandee Armee were French, with large contingents provided by Austrians, Germans and Poles.

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Which would be the most immediate consequences of his failure?

Capture of the Duchy of Warsaw, Russian counter-offensive, Prussia being involved in the coming war once again as a Russian ally.

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How did his contemporaries react to it?

It's hard for me to provide exact quotes as I never seriously researched this section of the war of 1812. Perhaps some other Redditors would prove to be more useful in this regard, but I imagine that many contemporaries were shocked as the campaign proved to be disasterous.

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Was it immediately perceived as his major defeat which had proved his hitherto perceived invincibility wrong? Did it crush him personally? Did it bring about his downfall?

Yes, it was a notable defeat, but the Napoleonic wars were far from over (French troops would again be a serious antagonist to Coalitional armies during campaigns of 1813-1815). Napoleon definitely wasn't broken as his energetic actions on reforming his army can show. He will later actively try to disrupt offensive of the Sixth Coalition with plans on waging war on enemy territory anew.

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How major of a damage had he nevertheless inflicted on Russia: economically, demographically and so forth?

It was enormous: many old Russian cities and villages have been burned to the ground, including Moscow, arguably second most important city in the Empire. Thousands of civilians were dead and wounded, either by accident or due to French plunder. Economical ties within the state were seriously disrupted, many people had nowhere to return to afterthe French had been expelled. Cultural damage proved to be as serious since many Russian architectural, literal or sculptural works were irrevocably lost in the fires and as a consequence of marauding actions of invading troops, notable mention belonging to the original text of The Tale of Igor's Campaign - medieval Russian epic poem burned by fire in Moscow.