Red Harvest, I have done a bit of thinking and digging into the matter myself some time ago.

I generally agree with you, but on the matter of the Hastati and Principes I do not.
It seems that the Hastati were in fact purpose made from the get go. Their name actually derives from, not the Hasta but the Hasta Velites, a throwing spear, as Hasta seems to be a rather young name for a thrusting spear.
Anyway, they came about with the reforms abolishing the phalanx in its most solid form. The Romans had seen how effective the Samnites and Celts had been with their heavy javelineers (in case of the Celts it was just the normal warrior). The Samnites set up in a dual-ascies as you said, one line with javelins and large shields (think for a second about how close that description is to the Hastati) and the other with spears. Apparently the Roamns saw the benefits of a lighter force that was able in melee.

The Hastati were meant to win smaller engagements with weak enemies, in that they disrupted with their javelins and then charged in with their swords hopefully breaking the enemy outright. Should that fail they would disengage right away and let the true melee warriors get to the fight, the Principes. The names says it all, the main troops. They were armed with spears like the Triarii as that had been what the Samnited had always been using and the Romans themselves were confident in (few warriors went into battle without a spear even if he had a sword).
Later, quite possibly in the fights against Pyrrhus, it was understood how effective the javelins were right before the charge, and that they were more versatile than the spear (swords make much faster work of phalangites if it can get within them). Lastly the spear is indeed not as aggressive as the sword, and the main fighting troops need to be aggressive and not let the battle develop before them, but rather develop the battle themselves. And thus came to be the Principes as we know them.

But all this is very much conjecture.