How strong is the Warrior of Light?

[Korbei]

You're the one that is wrong. The Sundering was AFTER life started springing from Zodiark's intervention

Yes but life that began after the sundering is not sundered. Only souls that existed at the point of the sundering are split and can be rejoined. There is way more life in the Source and its reflections than existed at that point. That's the fuel that powers Zodiark. There needs to be a surplus in order to return all the original souls while also rejoining the world. That's the trade off. All those extra souls get offered up in sacrifice.

I'm not sure I'd say 'very few' so much as 'not everyone'.

This is true, it's an assumption but the game only gives us a few specific unambiguous examples. And as stated above, there are way more souls extant in the present than in the unsundered world. Life is being grown in excess to feed Zodiark. The Ascians intend to return every sundered soul to its original form and the cost is great.

Outside of specific circumstances, like containing the light from light wardens, I'd say no.

Emet-Selch makes a very belabored point about how the WoL is different and better because they are seven-times-rejoined. (At that point). And to me it feels very reminiscent of the movie The One. Paraphrasing, he basically says let us finish our work and you'll become like a god.