English requires an article before knife: "A knife is also a weapon." or "The knife is also a weapon."
"Knives are also weapons" should also be acceptable.
Yes, I agree. Also, "That knife is also a weapon." should be accepted.
"Pisau itu" means that knife, so I think you are right and it should be changed.
It's got nothing to do with that “itu” though, “Knife is” wouldn't make sense regardless.