I can't comment on the statistical methods - that was not my best subject in school.
Númenor did not exist in the First Age; that land, and the extended lifespan of its inhabitants was a gift to a particular subset of Men, thanking them for their assistance during the War of Wrath:
It is said by the Eldar that Men came into the world in the time of the Shadow of Morgoth, and they fell swiftly under his dominion; for he sent his emissaries among them, and they listened to his evil and cunning words, and they worshipped the Darkness and yet feared it. But there were some that turned from evil and left the lands of their kindred, and wandered ever westward; for they had heard a rumour that in the West there was a light which the Shadow could not dim. The servants of Morgoth pursued them with hatred, and their ways were long and hard; yet they came at last to the lands that look upon the Sea, and they entered Beleriand in the days of the War of the Jewels. The Edain these were named in the Sindarin tongue; and they became friends and allies of the Eldar, and did deeds of great valour in the war against Morgoth.
To the Fathers of Men of the three faithful houses rich reward also was given. Eönwë came among them and taught them; and they were given wisdom and power and life more enduring than any others of mortal race have possessed. A land was made for the Edain to dwell in, neither part of Middle-earth nor of Valinor, for it was sundered from either by a wide sea; yet it was nearer to Valinor. It was raised by Ossë out of the depths of the Great Water, and it was established by Aulë and enriched by Yavanna; and the Eldar brought thither flowers and fountains out of Tol Eressëa. That land the Valar called Andor, the Land of Gift
The Silmarillion IV Akallabêth
The question discusses the possibility that the Númenórean Kings may have lived shorter lives (well, shorter than they would have otherwise) thanks to having violent deaths, but this isn't actually the case.
Númenor just wasn't involved in much fighting. The first major military conflict of the Second Age was the War of the Elves and Sauron, which was primarily fought by the Elves and Sauron (wouldn't you know it). The Númenóreans were very infrequently involved in that conflict; they helped to hold the Grey Havens, and utterly routed Sauron's forces in a subsequent battle:
In 1695, when Sauron invaded Eriador, Gil-galad called on Númenor for aid. Then Tar-Minastir the King sent out a great navy; but it was delayed, and did not reach the coasts of Middle-earth until the year 1700. By that time Sauron had mastered all Eriador, save only besieged Imladris, and had reached the line of the River Lhûn. He had summoned more forces which were approaching from the south-east, and were indeed in Enedwaith at the Crossing of Tharbad, which was only lightly held. Gil-galad and the Númenóreans were holding the Lhûn in desperate defence the Grey Havens, when in the very nick of time the great armament of Tar-Minastir came in; and Sauron's host was heavily defeated and driven back. The Númenórean admiral Ciryatur sent part of his ships to make a landing further to the south.
Sauron was driven away south-east after great slaughter at Sarn Ford (the crossing of the Baranduin); and though strengthened by his force at Tharbad he suddenly found a host of the Númenóreans again in his rear, for Ciryatur had put a strong force ashore at the mouth of the Gwathló (Greyflood), "where there was a small Númenórean harbour." [This was Vinyalondë of Tar-Aldarion, afterwards called Lond Daer; see Appendix D. p. 274.] In the Battle of the Gwathló Sauron was routed utterly and he himself only narrowly escaped.
Unfinished Tales Part 2: The Second Age Chapter IV "The History of Galadriel and Celeborn"
And that was pretty much it. Unfinished Tales also has a chapter devoted to the Kings of Númenor, and the only one who died in battle was Ar-Pharazôn, the one who tried to invade Aman.
Unnatural deaths likely increased following this, as the surviving Númenórean established the Kingdoms of Gondor and Arnor, but none of the people who we have confirmed lifespans for. Elendil and his Faithful, the ones who survived the destruction of Númenor, all lived to fight in the Last Alliance, which was the second (and last) major conflict of the Second Age.
The Third Age
This is when these unnatural deaths start coming in, as Isildur and his heirs consolidate their kingdoms and face resistance from the locals.
However, we also have to remember the fading grace of the Valar; the original Númenórean Kings were given a long lifespan, but that started to shorten as they became more and more hostile to the Valar and the Eldar:
In those days the Shadow grew deeper upon Númenor; and the lives of the Kings of the House of Elros waned because of their rebellion, but they hardened their hearts the more against the Valar. And the nineteenth king took the sceptre of his fathers, and he ascended the throne in the name of Adûnakhôr, Lord of the West, forsaking the Elven-tongues and forbidding their use in his hearing.
The Silmarillion IV Akallabêth
This fading wouldn't really stop until Aragorn took the throne. Although the lives of Men in the Third Age was much longer than we would expect from a medieval society, they were still shorter than in the Second Age, but with far fewer bloody and destructive wars than the First Age.