Re: lower back issue in heavy deadlifts
I would say that since your lower back is moving well out of position even before the slack is taken out of the bar that this is more mental than anything. You are initiating the lift by rounding off the low back, even before you've fully loaded up. That's the first thing that moves. It should be the last bodypart to show movement. You've got the wrong movement pattern going on here.
You need to lower the weight until you find a level that you can initiate the correct movement. Repeat that until it's ingrained. I would probably not let you go over 300, or even less, for a while.