1. We literally see smoke but I digress
2. Did he need the form to defeat the giant? No. Did he need it to oneshot the giant? Yes. The same luffy stated he may have needed g4 to take out ulti and this was the same luffy that obtained advanced ryou.
3. Bs logic. Both onepiece and dragonball operate with the premise of transformations and techniques. Haki usage is a technique while g5 is a transformation. Its retard logic to say that a technique at a person's weakest level is stronger than a person in their peek form (tranformation). This also applies to onepiece
4.
FORCED:
done or produced with effort, exertion, or pressure
obtained or imposed by coercion or physical power.
1
: compelled by force or necessity
2
: done or produced with effort, exertion, or pressure
Luffy felt it as a necessity to use G5 against awakened lucci. Therefore by definition he was "forced" into using it