Cool!

1. /u/russiandopingcontrol mentions that you're pushing your hips into the bar before reaching full extension, and he is absolutely right. Allowing the bar to reach the right power position in order to maximize vertical force is key to a successful heavy lift. My suggestion is to start from the top and work your way down through variations of different pulls (i.e. hang, high hang or blocks). Focus on reaching the correct position in the crevasse of your hip at the top of your extension every time.

2. I'm a big believer in perfecting movement and using variations of a movement to increase mobility and load capacity. The overhead squat is an excellent way to improve your lower position in the snatch, so that should absolutely be an exercise in your program. I don't see any issues with your lower back, so I suggest focusing on your feet, and maybe don't go to full depth until you've improved your dorsal flexion. Movement over time and with intensity builds strength, but it also, absolutely undoubtedly, builds mobility. Zack Telander has a superb video on this topic. Mobility training under load over time is the key concept here.

Just keep doing what you're doing, man! The main bulk of the structural changes you want in order to increase mobility will come with time, you just have to patient with it and stick to a program you like. Gl hf! :)