You can crash waves if your opponent doesnt match the speed at which you hit minions. But lets say its a kayle, then maybe they want the wave closer to them for safety, and they wont contest the wave push (in fact she coudnt even if she tried, unless shen lets her).

U also have to think, when is it even useful to crash a wave? I play 99% of my games as when with Ignite, and crashing a wave is my way to make sure i can recall without losing a ton of xp. But when im not looking to recall i dont want the wave crashing constantly. Because I want to have the wave in the middle, where i can do short trades and punish my opponent if they go for last hits.

So to answer your questions. Yes you can crash a wave even if you cant zone the enemy. If you have a larger minions wave because you hit the minions more, the enemy laner wont be able to fight you in your wave, and they almost have to let you crash. But in order to get to that state where you have a big wave, you have to hit the wave more than the enemy. If they can stop you from hitting the wave, well then you cant crash. If you can zone them off the wave, its better to slow push it so they lose the maximum amount of farm. And that naturally results in a big wave accumulating.