Evocation does not restore mana.
AND
Evocation does not regenerate mana.
Evocation increases the amount of mana that is regenerated by other methods that do regenerate mana.
Try this. Stand in a capital city and cast spells until you have too little mana to do anything. Now … just stand there. Do nothing. What happens to your mana bar?
It refills on its own. Every couple seconds a little bit more is added.
That is passive out-of-combat mana regeneration. Every mana-using character in WOW has that.
The stat SPIRIT, both the amount your character has without gear, and the amount available on gear, increase that passive regeneration. There’s plenty of sources out there on the formulas.
So, using a practical example, I’ve got a level 22 mage with 1440 mana and 54 spirit, and they regenerate about 30 mana every 2 seconds WITHOUT using Evocation.
Evocation does not regenerate mana.
Now, they cast Evocation, and instead of 30 mana, when it goes to regenerate at 2 seconds they get around 400 mana. If they’re still channeling 2 seconds later, another 400 mana. Stop channeling (early) and 2 seconds later they get around 30 mana.
Evocation did not cause the regeneration. Evocation increased the amount. Ending Evocation stopped increasing the amount.
As for the difference between restore and regenerate, not really relevant and a red herring.
Those mechanics in game that give something only to take it away when the channel or action is done tend to explicitly say so - such as Warrior’s Last Stand. The current and max health is increased for 20 seconds, but when that ends, the increase goes away. (Off the top of my head, I don’t remember any abilities that temporarily increase mana - either current or max.)