Hey, welcome back to gunz!
Here are my opinions to your questions
1. This is what I call finger rust and since I've also taken long breaks from gunz I know with a little bit of effort you can easily regain your former speed, It's like riding a bike, once you learn how to you will never forget, but the balancing takes some time to re-learn. In my opinion your keybinds don't matter, I've seen skilled players with crazy hard keyboard combinations, I personally use Q as flip, R as reload, E as block, 1, 2 and 3 to switch between weapons. (I made a video to show-case my alien keybinds https://www.youtube.com/watch?v=FYfbotHTbKA)
2. Some keybinds give you certain advantages in terms of movement, but I still believe that keybinds are not too important, for example with my weird keybinds I cannot walk left and reloadshot since I am holding A with the finger I reload with. What I meant by not too important is, there is no such thing as "perfect keybinds", everyone uses their own "freestyle", focusing on keybinds is more time consuming than just practicing your current ones.
3. TBF is useless, you can easily learn it in 3~ days with enough practice, but it's a waste of time, if you can DBF consistently that's all you need.
4. Insta is definitely a good skill to learn and you can ask someone to train it with you, make sure to buy the "Training Sword" which deals 0 dmg.
My advice to you is to just use training sgs and jump around and shoot around in a training room, the purpose of this is to train muscle memory and to warm up your fingers to complex combinations not to certainly practice how you would play in a real competitive room.
It's not that you're low level, its probably because you:
a) look noobish
b) you don't have high level shotguns, or donor shotguns
c) you are too passive
d) you play on male
If you don't know what donation shotguns are, they are shotguns you can buy for real money, but they give you a huge advantage compared to your low level shotties that you're using right now.
If you plan on buying them, buy these two ones.
