1. It was theorized that light could be a wave way before the double slit experiment. Like, a century before. So no, it wasn't "assumed light is 100%" quantized before that experiment.

2. Anything that is a wave can be cancelled, so this idea was baked right into the wave theory of light, they just didn't have the ability to control light precisely enough to prove it until the double slit experiment. You don't need quantum mechanics to explain wave theory, it just happened that the double slit experiment, while proving that light behaved like a wave, also showed other characteristics that it was also behaving in a quantized fashion. The fact that light is quantized into photons has nothing to do with the fact that they cancel so you really don't need quantum mechanics to explain it. The reason light can be cancelled is exactly the same as every other thing in physics that behaves like a wave.

3. The word quantum comes from the word quantization not "quantify". Those two words mean different things

4. Light is a wave. It also happens to be a particle. So the "existence of waves" is not a different subject. It's exactly this subject

Edit: Love the snarky edit to a post full of being confidently wrong. I'm going to go engage with others. Good day, sir/ma'am!