So upon making skills legendary the level gets reset to 15. The perks are then able to be reallocated, does this mean that those perks are then wiped away and effectively "lost" to the player or are they still active?

I am not sure because I have yet to legendary any skills yet, but I understand it as the perk points are refunded and the skill is reset. Thus the previously perked skills are wiped and you reallocate those perk points along with the additional perk points you gain by continuing to level.

I think that's how it goes anyways.

So then if I had conjuration at lvvl 100, made it Legendary and had the perk for dual conjuration, I would no longer ba able to dual-cast? In your opinion that is

They are lost, you have to gain them back again.

That's why its always best to use it on skills you don't use.

Dang I was afraid of that.