Is my assumption right, that i can only crit while having "Primordial Insights"?
Yes, but it will only "double" your damage, even if u have "+ Critical Damage", your original spell will benefit from it, but not the ailtment.

Will "% Material Damage" scale my rend ailment damage?
Yes.

Will it scale in any kind or way with my weapon damage (the total and/or the corresponding dmg type on it)
Don't think so, at least I couldn't test it. only thing u can do is provide an specific type of ailment, according to your weapon damage type .

And what about the skills damage that applied the ailment?
The skill itself no. But some skills have bonus like "The ailment applied with this spell receive a bonus" or something like that.
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I'm trying to create an full mage build, with ailments.. but don't know if will workout either.