They explained to you already that nobody really knows what the Echo is capable of, and that part of their job is to study it and find out. I'm also not sure about the 'makes you stronger' part... I more got the feeling that the ability to manipulate aether, something the Garleans lack, made Zenos stronger (and Fordola, too, looked to me like she was unable to do that in any meaningful way before). Also, we had other instances of people turning into primals before, and while the circumstances there were different (they summoned the primal themselves, inside their bodies), I feel it's actually the same mechanism at its bottom. As in, it's more about your force of will helping you control the primal, less the Echo. The Echo just makes it that you actually have a chance to pitch your will against the primal, by preventing them from tempering you. Arenvald seems to be a pretty good example of this, since he is nowhere near our level of strength despite bearing the Echo. He isn't the only one, either, btw, there are (or were, some died in the Waking Sands ambush) quite a few more Echo bearers in-canon.
Also, I feel you guys are confusing the Echo and the Blessing of Light, which seem to be two different pairs of shoes. The Echo basically everyone might have; but the Blessing is what sets us appart from all the other Echo bearers. That's made quite clear, I think, with Ysayle. We always knew she had the Echo, but we only found out she had the Blessing, too, when she used it to sacrifice herself for our cause.
That being said, yes, there are some points that seem to imply the WoL isn't quite human anymore, or never was... but then, shouldn't Midgardsormr, for example, know that? And the Ascians? If we truly aren't mortals (anymore), then why are they still calling us that?
