The whole point of the book will be defeated if Voldemort survives - whatever happened to the good always wins over evil thing? JKR seems to be a huge supporter of that.
Whether Harry will die or not, I'm still unsure. If he lives, it's after killing Voldemort. If he dies, he takes Voldemort with him. But Voldemort is going to die.
I can see where you're coming from that Harry's underage and a bit immature and not that powerful - but Harry has it in him, I'm sure. I guess he just had to rely on people too much to let it show. He's all alone now - he's got to do it for himself.
However it ends, I'm sure it'll be great. Jo knows what she's doing - she won't let us down.