We know such power can and has been granted, due to Garland's existence since the dawn of Gaia. Also, considering the level of power Garland required in order to assimilate worlds, it's not impossible for him to grant immortality to another, should he will it.

So yes, Zidane can technically live for all time.

Garland never died until Zidane reached the Crystal World. He projected his words telepathically to Zidane with his last ounce of strength.

It's evident he has the ability to teleport, however it's probable this ability, even for Garland, is limited. He's most likely in a section of Terra which wasn't obliterated by Kuja.
That is what immortality means, yes.

if Zidane is immortal then I wanna see a sequeal where zidane is the last one left and drowns in his sorrow! xD

There's absolutely no relation between Necron and... well, anything. As far as the player is concerned, he was just thrown in as an extra boss. Considering Garland wished to retain life on Terra, having connections with Necron, of whom wishes to restore everything to the "zero world," (by destroying life) would result in major hypocrisy.

I can't see how you possible made that assumption. It's absurd.

Yet there is a detailed analysis of Necron's possible relationship with the Iifa tree.
http://www.gamefaqs.com/console/psx/file/197338/36121