1) I’m not sure, but you might want to change the method in which you clear floats. I use this one: http://www.adamkaplan.me/css-clearfix/ – if that doesn’t work, it might be time for a reduced case CodePen.
2) The rule of thumb is to start at mobile sizes, as you scale up and the content looks funky, create a media query to compensate for the extra space. Continue until you’ve reached your max-width, etc.
3) That would just give you a single break point, it probably won’t solve most of your problems at all sizes, but it’s a start.