They can be killed as the C'tan picked each other off until there were only four of them remaining. So not invincible, but as close to it as possible.
thinking to it, if i should appear in the setting of 3d edition codex, i would have 4 C'tan with an army of slaves.
which i recall it's the only and true one, despite your nostalgia and your insistence to pretend that a question with a related picture of actual 42° millenium could instead refer to a previous, largely retconnected version. It has been fun for a while to play your game, but i'm gonna put it to rest.
Like said, it's all make-believe. People are free to play whatever edition they like. fixed that for you He who makes the post makes the rules! You don't interpret Warhammer rules based on the pictures of the models in the army book / codex, why should you do differently here? It has gone on for much longer than expected. It's probably best to move on.
Respectively Inquisitor Eisenhorn and Inquisitor Amberley. The first one is the protagonist of a famous trilogy by Dan Abnett, and he also received a miniature: for a little while there was the rumor that Cavill would have played his role in the still to come warhammer movie.
Inquisitor Amberley is known mostly because it's a character and the main narrator in the novels of CIAPHAS CAIN HERO OF THE IMPERIUM (capital letters required). Which is a really fun take on the seriousness of the setting, as Commissar Cain is officially known as one of the greatest hero of the imperium, while in truth he would just be in a safe position with no dangers and few responsibilities. Of course, when he finds himself in dangerous situations, he's forced to save his hide, which results in him saving also the day, which inflates his status even further and increases the chances of dangerous assignments.
That would be a waste, I could play the God-Emperor myself. All i need is just some makeup to resemble a sitting corpse.
This whole argument is moot. The question is not how well your army would fare, but how well YOU would fare if your army was surrounding you in the 40K setting, and as a general rule, if you're a human surrounded by Necrons, you are screwed no matter how you slice it.