I actually think the bar isn't low enough that that's the start of the problem (like 1-2 inches high) and your elbows are too high and you are forced to over correct and exaggerating their role - once it starts getting heavy you are going to hurt yourself, and I bet you are already quite uncomfortable and not actually that stable - it's resting on your arms not your shoulders right now and you can see the bar rolling and wiggling around.
Reread the bar placement section, get it lower and keep your shoulders tight, elbows up and tight but roughly neutral with your torso. Gravity and your hands will clamp it down like it's in concrete, you'll be tight and the bar won't be going anywhere but you won't feel like you are a pretzel either.
Your chest seems OK to me, and I think your focus on it is causing you to lose hip drive. Just keep it tight and pivot from your hips down and up. Correct bar placement will probably correct this also.
I have long forearms too, try wrapping your thumb around and see if makes your life easier - if your shelf is still rigid and you are doing everything else right who cares, having said that if you correct your bar placement/angle issues it very well might resolve itself organically.
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