I accept all those examples except the last: I believe the historical record has it that King Alexander I Karageorgevich of Yugoslavia had a younger brother who died at birth -- or shortly afterward. Sadly, his mother (born Princess Zorka of Montenegro) ended up dying of complications involving that birth; so he never knew her.
In this, Alexander would be exactly like Henry VIII of England and George V of Great Britain: all three kings were second sons followed in birth by brothers who died in infancy.
The whole point in my stressing the status of being the LAST-born son in the family, to qualify as youngest, is that royal history is full of vagaries. Wars, revolutions, usurpations, treaties, deaths (accidental or natural) and a host of other factors can lead to younger sons in general unexpectedly succeeding.
In today's world, however, unless one is the HEIR, he doesn't stand a reasonable chance of succeeding. As such, not even a second son can expect to ever sit on a throne as a reigning monarch: the man we know in history as King Haakon VII of Norway would NEVER have become one, but for being proferred the throne after the country separated from Sweden and elected him to start a new royal dynasty.
We have seen how Prince Andrew was born second in the line of succession to the British throne -- only to find himself today a remote ninth. But in the past, it wasn't entirely unheard of for even third, fourth, or fifth sons succeeding. That was why I focused on last sons -- notwithstanding the fact that such persons could have been second sons.
If indeed the last son was a second son, it would have meant that the dynasty was potentially at risk of extinction -- since succession laws have traditionally been restricted to male heirs in the male line (the exceptions being England, Scotland, Spain, and Portugal). As it was, France has had numerous cadet branches breaking off from the original Capetians.
I realize I might have been mislead by "youngest" sons by "younger" sons.
Would you accept King Otto of Bavaria, youngest son of King Max II Josef ?
Manuel II of Portugal.
Pedro II of Brazil.
Paul I of Greece.
Michael Romanov, emperor for 1 day after Nicolas II abdication.
Alexander I of YU.
That France had numerous cases is perhaps no surprise, given the succession law in operation. Perhaps not all the kings you mentioned were, strictly speaking, *youngest* sons in their families. But you're quite right that a third or fourth son comes close to being one.
If it came to that, King Albert II of the Belgians might qualify as a youngest son who succeeded -- if one were to restrict discussion to the issue born of only the first marriage of his father, King Leopold III. As it was, he got a half-brother (Prince Alexandre) from his father's second marriage. So perhaps he doesn't quite count.
What I find eerie is that Portugal, despite operating under male-preference primogeniture (as opposed to the Salic law), exhausted the multiple sons of Queen Maria II: she has no legitimate representative through any of them today. And because her daughters excluded themselves from the succession by moving to foreign lands to marry, the royal succession eventually passed to the descendants of her uncle, Dom Miguel. But of course, the monarchy got abolished by then.