That would mean no older support. I suspect you probably want:

if ( $dir == 'newer' ) {
$this->addWhereRange( 'cat_pages', 'newer', $min, $max );
} else {
$this->addWhereRange( 'cat_pages', 'older', $max, $min);
}

I don't understand what you mean. Both mine and your code seem to work exactly the same to me, whether I use acdir=ascending or acdir=descending.

The only difference is that my code produces something like (with acdir=descending&acmin=0&acmax=3):

WHERE (cat_pages>='0') AND (cat_pages<='3')

while your is:

WHERE (cat_pages<='3') AND (cat_pages>='0')

But I guess I'm missing something.

They probably do, but hard coding the direction isn't probably the best idea... It might end up having some unexpected consequence in future...