If one or more leaders of a NATO country are killed or injured by an attack while outside of NATO (e.g. in Kyiv), is that considered an attack on NATO, and therefore a trigger Article 5?
In theory, no. If occupying a British dependent territory outside the NATO area does not qualify, why should hitting a train count?
However, Article 5 will mean whatever the NATO governments want it to mean. It was invoked over the 9/11 attacks on New York and Washington, which was not an inter-state act of war in the traditional pattern. Yet NATO wanted to make a point and used Article 5 to respond. Like all deterrence, NATO depends on the faith in the political will of the member governments, as well as facts on the ground like tripwire forces and joint pre-planning.
The EU could publicly send three heads of government to Kiev and dare Russia to attack. Russia could attack them and dare NATO to respond.
It would be up to the country the leader of which would be killed. Certainly there is precedent for considering an assassination to be an act of war. The killing of Archduke Franz Ferdinand by non-state actors was treated as an act war and triggered World War I.
But there is a more recent example as well. President Clinton ordered bombing of Iraq in 1993 in response to Iraqi government's failed plot to assassinate former US President George H.W. Bush. From the article:
Madeleine Albright, the U.S. ambassador to the United Nations, told the U.N. Security Council that the attack “was designed to damage the terrorist infrastructure of the Iraqi regime, reduce its ability to promote terrorism, and deter further acts of aggression against the United States.”
Which implicitly makes the claim that the failed plot was an act of aggression against the United States.
The case for treating the killing of a leader of a country by a state actor is more, rather than less, likely to be treated as an act of war. But it would be the choice of the country whose leader was killed.
Article 5 is triggered when a nation requests help due to having been attacked. An attacked nation doesn't have to request it. But if there is legitimate basis for treating an event as an act of war, the request cannot be denied.