1) This is due to weakness of your Vastus Medialis muscle (tear drop) and Vastus Lateralis (External quad sweep). You just need to strengthen them over time. As long as you are not doing too much weight that you feel pain or stress on the inner part of the knee, you are OK. However, it is important you strengthen BOTH the quads and hamstrings to ensure a strong and stable knee joint. Over time if you do not fix this muscle weakness in your legs, you could damage your knees as they should not track inward when doing pressing/squatting movements.
2) The best way to exercise any muscle groups is to use a weight you can handle with proper form. It doesn't matter if you use DBs or BBs because over tiem you will get stronger. Standing and pressing overhead with either a DB or BB is still and overhead press. When you sit down, it becomes a seated press.
3) The most effective to build total back strength and size is not to rely solely on cable machines. When you do rows of any type with DBs or BBs and Pull-ups, you work not only the Lats but also all the secondary stabilizing muscles. When you do machines or cables, you only train the targeted muscle and do nothing for the stabilizers. The best course of action is to mix in both free bar rows and pull-ups with cable back work.
4) The best all around back exercises are pull-ups and rows.
John