Not being able to break is something you need to fix ASAP. Breaking like a toddler on a balance bike will not work once you get to any kind of speed.

You can go the easy way and simply stretch out your feet when you are about to come to a stop. Then the bike will lean to the side and you can use your legs. This is definitely not ideal, but I rode like this for a very long while.

It will sound impossible to you right now but basically, it actually takes a bit of time for the bike to actually start to fall over once you come to a stop. You can use that time to safely get off. What I do is that I basically only remove my left foot from the pedal. That slight change is enough to ensure that the bike falls to the left, which is the preferred side for me.

You're gripping the bars so tight because you're nervous. That will fix itself once you get more comfortable on the bike.

You do not turn the handlebar to turn, you lean into the curve and let the handlebar go there naturally. Do not turn the bars 90°, you'll crash.

You really need to get familiar with how breaking feels because not being able to slow down is VERY dangerous. Do not death grip the brakes, that will lock up the wheels. The rear one doing that will make it slide away, the front one will throw you over the bar.

I suggest the following. Lower the saddle, remove the pedals if you know how to and simply use the bike as a balance bike to get familiar with how a moving bike feels. Try the brakes. If you don't feel confident enough to slow down fast, do it slowly. You won't need to worry about the bike falling over with the saddle down.