Regarding the rounding of the lower back, have you watched Mark's video on lower back position control? I've been squatting for a solid two months thinking I was tightening my lumbar, but I wasn't. I only realized this last night when I watched the video. You might also want to watch the bar position video. It looks the bar might be a bit lower than low bar position (another error I was making).
It does look like you're improving though. Kudos to you for the hard work.