Answer 1

The best practice comes to use the right thing in the right situation:

0 for numbers
'\0' for characters
NULL for pointers
false if you are using stdbool.h

Answer 2

~ swim ~ NULL is (*(void)0) right

Answer 3

I'm not sure but my opinion is that '\0' is the ending element of any character array(string) but the NULL is value that every variables can have.
if a variable's value be NULL, it means that the variable has been initialized but hasn't any value; or has the value of 0.