Well, im not an expert, but ill throw my two cents in here.
The squats look fine to me, but if you want to nit-pick there's a few things. You may might be going lower than you need to which can lead to that lumbar flexion you see in vid #1. As for knees sliding forward, if you're concerned about it then focusing on pushing the knees out can be beneficial. Your stance might also be a bit wide, but there's no point changing it if it seems to be working for you. As for the lower back rounding, using the forum search function for the unfortunate term 'butt wink' will give your answer about that.
If any of the more experienced guys have anything to add or correct, please do.