You're not following. You can have as much dash cam footage you want of you rear-ending somebody who is making a lane change, and you'll still be at fault. You will be found guilty of travelling at an unsafe speed and/or unsafe distance. It's that simple. Whatever you think you're allowed to do - "Protect your lane," etc. - you actually aren't legally allowed to do.

Again, you will have to show something more than somebody cutting you off. There will need to be negligence on the part of the lead driver. If you hit their quarter panel, that's one thing. But if you hit them square in the rear, the burden of proof doesn't just lie in the dashcam footage of you getting cut off - it lies in you proving that the other driver engaged in actual negligence.