how is possible for sylar to survive when he has had a samurai sword go straight through his heart??
a couple of petty stiches shouldnt, realistically, be able to keep him alive

But didnt sylar lose all his powers after the attack? Rendering him unable to "heal" with his powers. Plus candice said he needed 8 more operations so why is he still alive now with him not having those operations?

He did have the operations, she said that after 8 operations he made it.

Plus, he had the virus cause the company injected him.

That's pretty correct, I mean, HRG shot him, and he recovered by himself.