Bash scripting questions:
 

1. Is the $# var not global? I can't seem to access it from within functions() in my script.

2. What is the difference between (( expression )) and [[ expression ]] my tutorial states that they are opposites however (( 5 > 9 )) evaluates to 1 (false) which is same with [[ ]]

iirc, historically [[ ]] came first, for logical ie if/then tests.
(( )) came later in ksh and bash for 'built-in' arithmetic calcs, to avoid calling another (external) util eg expr.
However, (( )) will also return a true/false equiv under certain conditions (as per your example).
HTH

No, $# is not global. Inside a function $# contains the number of arguments passed to the function, not the numbers of arguments passed to the script.

you could do it by passing the scripts args again to the function

func "$@"

though of course they are now local to the function

so any changes made to the args will not propogate backwards to the caller

so a waste of time really