1. No
2. I'm not certain, but I've a strong feeling the answer is no.
3. Makes no odds, but if you have access to a computer and printer, it can't hurt to type it for the sake of legibility.
4. It sounds a bit weak to me - it leaves open a question as to what is your original will, ad doesn't deal with testamentary capacity. It would probably be okay, but I'd want better than probably when dealing with my will. To ensure you get it right, I'd consult a solicitor. A codicil going wrong could lead to a lot of hassle later. You might be consider drafting the entire will again, revoking the current one, of course. You can just copy the construction of the original will.